As filed with the Securities and Exchange Commission on November 6, 2003

Registration No. 333-          

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-3

REGISTRATION STATEMENT

UNDER

SECURITIES ACT OF 1933

MYKROLIS CORPORATION

(Exact name of registrant as specified in its charter)

Delaware	04-3536767
(State or other jurisdiction of incorporation or organization)	(I.R.S. Employer Identification No.)

129 Concord Road, Billerica, Massachusetts 01821

(978) 436-6500

(Address, including zip code, and telephone number, including area code, of registrant’s principal executive offices)

Peter W. Walcott, Esq

Vice President & General Counsel

Mykrolis Corporation

129 Concord Road

Billerica, Massachusetts 01821

(978) 436-6500

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copies of all communications, including communications sent to agent for service, should be sent to:

DAVID B. WALEK, ESQ.

Ropes & Gray LLP

One International Place

Boston, Massachusetts 02110

Telephone: (617) 951-7000

Telecopy: (617) 951-7050

Approximate date of commencement of proposed sale to the public:    From time to time after this Registration Statement becomes effective.

If the only securities being registered on this Form are being offered pursuant to dividend or interest reinvestment plans, please check the following box.    ¨

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, other than securities offered only in connection with dividend or interest reinvestment plans, check the following box.    x

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.    ¨

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.    ¨

If delivery of the prospectus is expected to be made pursuant to Rule 434, please check the following box.    ¨

CALCULATION OF REGISTRATION FEE

Title of Each Class of Securities to be Registered	Proposed Maximum Aggregate Offering Price (1) (2) (3)		Amount of Registration Fee
Debt Securities
Common Stock, par value $.01 per share (4)	$	200,000,000	$	16,180

(1) There are being registered under this registration statement such indeterminate number of shares of common stock of the registrant, and such indeterminate principal amount of debt securities of the registrant, as shall have an aggregate initial offering price not to exceed $200,000,000. If any debt securities are issued at an original issue discount by the registrant, then the securities registered shall include such additional debt securities as may be necessary such that the aggregate initial public offering price of all securities issued pursuant to this registration statement will equal $200,000,000. Any securities registered under this registration statement may be sold separately or as units with other securities registered under this registration statement. The proposed maximum initial offering price per unit will be determined from time to time by the registrant in connection with, and at the time of, the issuance by the registrant of the securities registered under this registration statement.

(2) Not specified with respect to each class of securities to be registered by the registrant pursuant to General Instruction II.D to Form S-3.

(3) Estimated solely for the purpose of calculating the registration fee pursuant to Rule 457(o) under the Securities Act of 1933. No separate consideration will be received for any securities registered hereunder that are issued upon exercise, conversion or exchange of debt securities registered hereunder.

(4) Including such indeterminate number of shares of common stock as may from time to time be issued upon exercise, conversion or exchange of debt securities registered hereunder, to the extent any of such debt securities are, by their terms, convertible into common stock.

The registrant hereby amends this registration statement on such date or dates as may be necessary to delay its effective date until the registrant shall file a further amendment which specifically states that this registration statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933 or until this registration statement shall become effective on such date as the Commission, acting pursuant to said Section 8(a), may determine.

The information in this prospectus is not complete and may be changed. We may not sell these securities until the registration statement filed with the Securities and Exchange Commission is effective. This prospectus is not an offer to sell these securities and it is not soliciting an offer to buy these securities in any jurisdiction where the offer or sale is not permitted.

Subject To Completion, Dated November 6, 2003

$200,000,000

MYKROLIS CORPORATION

Debt Securities and Common Stock

We may offer and sell the securities listed above with an aggregate offering price of up to $200 million in connection with this prospectus. We will provide the specific terms of these securities in one or more supplements to this prospectus. You should read this prospectus and any prospectus supplement carefully before you invest.

Our common stock is listed on the New York Stock Exchange under the symbol “MYK.” The applicable prospectus supplement will contain information, where applicable, as to any other listing on any securities exchange of the securities covered by this prospectus supplement.

We may sell the securities directly to investors, through agents designated from time to time or to or through underwriters or dealers. See “Plan of Distribution.” If any underwriters are involved in the sale of any securities in respect of which this prospectus is being delivered, the names of such underwriters and any applicable commissions or discounts will be set forth in a prospectus supplement relating to those securities.

You should carefully consider the risk factors contained in our Quarterly Report on Form 10-Q for the quarterly period ended September 27, 2003 before making your investment decision.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

This prospectus may not be used to offer or sell any securities unless accompanied by a prospectus supplement.

The date of this prospectus is                     , 2003.

TABLE OF CONTENTS

	page
Mykrolis Corporation.	1
Risk Factors	1
Forward-Looking Statements	1
Where You Can Find More Information.	2
Ratio of Earnings to Fixed Charges	3
Use of Proceeds.	3
Dividend Policy	3
Description of Debt Securities	4
Description of Capital Stock	12
Plan of Distribution.	16
Legal Matters.	18
Experts.	18

ABOUT THIS PROSPECTUS

This prospectus is part of a registration statement that we filed with the Securities and Exchange Commission using a “shelf” registration process. Under this shelf registration process, we may, from time to time, sell debt securities and common stock, either separately or in units, in one or more offerings, up to a total dollar amount of $200,000,000.

This prospectus provides you with a general description of the securities we may offer. Each time we sell securities, we will provide a prospectus supplement that will contain specific information about the terms of that offering and the offered securities. The prospectus supplement may also add, update or change information contained in this prospectus. Any statement that we make in this prospectus will be modified or superseded by any inconsistent statement made by us in a prospectus supplement. You should read both this prospectus and any prospectus supplement, together with additional information described under the heading “Where You Can Find More Information,” before making your investment decision.

Unless the context otherwise requires, references in this prospectus and the accompanying prospectus supplement to “we,” “us” and “our” refer to Mykrolis Corporation and its subsidiaries.

You should rely only on the information contained in this prospectus and the applicable prospectus supplement. We have not authorized anyone to provide you with information different from that contained in this prospectus and the prospectus supplement. The information contained in this prospectus is accurate is accurate only as of the date of this prospectus, regardless of the time of delivery of this prospectus or of any sale of our securities.

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MYKROLIS CORPORATION

We are a worldwide developer, manufacturer and supplier of liquid and gas delivery systems, components and consumables used to precisely measure, deliver, control and purify the process liquids, gases and chemicals, as well as the deionized water, photoresists, other consumables, and vacuum systems, that are used in the semiconductor manufacturing process. In addition, our products are used to manufacture a range of other products, such as flat panel displays, high purity chemicals, photoresists, solar cells, gas lasers, optical and magnetic storage devices and fiber optic cables. Our products include membrane and metal based filters, housings, precision liquid dispense filtration pumps, resin based gas purifiers, polymeric brush rollers and mass flow and pressure controllers. These products are used by our customers in manufacturing operations to remove contaminants in liquid and gas processes, to purify liquids and gases, to clean wafers during chemical mechanical planarization operations, to measure and control flow rates and to control and monitor pressure and vacuum levels during the manufacturing process. We sell our products worldwide through a direct sales force and through distributors in selected regions. Additional information concerning us is included in our reports and other documents incorporated by reference in this prospectus. See “Where You Can Find More Information.”

We are a Delaware corporation that was incorporated in 2000 as a wholly-owned subsidiary of Millipore Corporation, a Massachusetts corporation, to operate Millipore’s microelectronics business and to be spun-off to Millipore’s shareholders. This spin-off was completed in February 2002. Prior to March 31, 2001, our business had been operated for over 25 years as a fully-integrated business unit of Millipore. Our executive offices are located at 129 Concord Road, Billerica, Massachusetts 01821. Our telephone number is (978) 436-6500.

RISK FACTORS

Investing in our securities involves risks. Please see the risk factors described in our Quarterly Report on Form 10-Q for the quarterly period ended September 27, 2003, which is incorporated by reference in this prospectus. Before making any investment decision, you should carefully consider these risks as well as other information that we include or incorporate by reference in this prospectus or a prospectus supplement. In addition, risks and uncertainties not currently known to us or that we currently deem immaterial may also affect our business operations.

FORWARD-LOOKING STATEMENTS

The Securities and Exchange Commission encourages companies to disclose forward-looking information so that investors can better understand a company’s future prospects and make informed investment decisions. This prospectus includes and incorporates by reference, and the accompanying prospectus supplement may include, “forward-looking statements” within the meaning of Section 27A of the Securities Act of 1933 and Section 21E of the Securities Exchange Act of 1934, enacted as part of the Private Securities Litigation Reform Act of 1995.

Words such as “expect,” “anticipate,” “approximate,” “believe,” “plan,” “estimate,” “intend,” and “hope” and variations of such words and similar expressions are intended to identify such forward-looking statements. In addition, any statements that refer to expectations, projections or other characterizations or future events or circumstances, including any underlying assumptions, are forward-looking statements. We intend such forward-looking statements, all of which are qualified by this statement, to be covered by the safe harbor provisions for forward-looking statements contained in the Private Securities Litigation Reform Act of 1995 and we are including this statement for purposes of complying with these safe harbor provisions. We have based these statements on our current expectations and projections about future events. These forward-looking statements are not guarantees of future performance and are subject to risks and uncertainties that could cause actual results to differ materially from those projected in these forward-looking statements. These risks and uncertainties include those set forth under “Risk Factors.” We caution you not to place undue reliance on the forward-looking statements.

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Except for our ongoing obligations to disclose material information under the federal securities laws, we are not obligated to publicly update or revise any forward-looking statements, and we expressly disclaim any obligation, to update any forward-looking statements, whether as a result of new information, future events or otherwise. In light of these risks, uncertainties and assumptions, the forward-looking events discussed or incorporated by reference in this prospectus or the accompanying prospectus supplement might not occur.

WHERE YOU CAN FIND MORE INFORMATION

We are subject to the informational requirements of the Securities Exchange Act of 1934 and, therefore, we file annual, quarterly and current reports, proxy statements and other information with the Securities and Exchange Commission. You may read and copy any document that we file at the SEC’s public reference room at 450 Fifth Street, N.W., Room 1024, Washington, D.C. 20549. Please call the SEC at 1-800-732-0330 for more information about its public reference rooms. Our SEC filings are also available to the public from commercial document retrieval services and at the web site that the SEC maintains at “http://www.sec.gov.” Our common stock is listed on the New York Stock Exchange, and you may also inspect and copy our SEC filings at the offices of the New York Stock Exchange located at 17 Wall Street, New York, NY. This prospectus is part of a registration statement on Form S-3 that we have filed with the SEC under the Securities Act of 1933. This prospectus omits part of the registration statement, as permitted by the rules and regulations of the SEC. You may inspect and copy the registration statement, including exhibits, at the SEC’s public reference rooms or from its web site.

The SEC allows us to “incorporate by reference” the information we file with them, which means that we can disclose important information to you by referring you to those documents. The information incorporated by reference is an important part of this prospectus and information that we file later with the SEC will automatically update and supersede this information. Accordingly, we incorporate by reference the documents listed below and any future filings we make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Securities Exchange Act of 1934 prior to the termination of this offering:

• Our annual report on Form 10-K for the fiscal year ended December 31, 2002;

• Our quarterly report on Form 10-Q for the quarter ended March 29, 2003;

• Our quarterly report on Form 10-Q for the quarter ended June 28, 2003;

• Our quarterly report on Form 10-Q for the quarter ended September 27, 2003;

• The description of our common stock contained in our registration statement on Form 8-A, dated August 10, 2001, including any amendment to that form that we may file in the future for the purpose of updating the description of our common stock; and

• Our definitive proxy statement dated March 28, 2003, filed in connection with our 2003 annual meeting of stockholders.

You may request a copy of these filings at no cost by writing or telephoning the office of Bertrand Loy, Mykrolis Corporation, 129 Concord Road, Billerica, Massachusetts 01821, telephone (978) 436-6500.

You should rely only on the information incorporated by reference or provided in this prospectus, any prospectus supplement and the registration statement. We have not authorized anyone else to provide you with different information. We are not making an offer of these securities in any state where the offer is not permitted. You should assume that the information in this prospectus and any prospectus supplement, or incorporated by reference, is accurate only as of the dates of those documents. Our business, financial condition, results of operations and prospects may have changed since those dates.

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RATIO OF EARNINGS TO FIXED CHARGES

The ratio of our earnings to fixed charges is set forth below for each of the periods indicated.

	Nine Months Ended September 27, 2003		Fiscal Year Ended December 31,
			2002		2001		2000	1999	1998
Ratio of earnings to fixed charges (a)	—	(b)	—	(b)	—	(b)	24.4x	2.1x	—	(b)

(a) These ratios are calculated by dividing (a) earnings before income taxes plus fixed charges less income (loss) from equity investees and minority interests, by (b) fixed charges. Fixed charges include interest expense, the portion of interest expense under operating leases we deem to be representative of the interest factor, and capitalized interest.

(b) For the nine months ended September 27, 2003 and fiscal 2002, 2001 and 1998, earnings before income taxes plus fixed charges less income (loss) from equity investees and minority interests were insufficient to cover fixed charges by $7.2 million, $29.0 million, $41.1 million and $43.1 million, respectively.

For purposes of calculating the ratios of earnings to fixed charges, (i) fixed charges consist of interest on debt, amortization of discount on debt and capitalized interest, and (ii) earnings consist of pre-tax income from operations and fixed charges (excluding capitalized interest) and include the amortization of capitalized interest.

USE OF PROCEEDS

We intend to use the net proceeds from the sale of the securities offered by us pursuant to this prospectus and any prospectus supplement for our general corporate purposes, which may include cash for acquisitions, the expansion of our manufacturing facilities and other capital expenditures and working capital. The prospectus supplement relating to a particular offering of securities by us will identify the use of proceeds for that offering.

DIVIDEND POLICY

We have never paid a cash dividend on our common stock and do not anticipate paying any cash dividends in the foreseeable future.

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DESCRIPTION OF DEBT SECURITIES

We may from time to time offer and sell debt securities (the “Debt Securities”), consisting of debentures, notes and/or other unsecured evidences of indebtedness. The Debt Securities will be either our unsecured senior debt securities (the “Senior Debt Securities”) or our unsecured subordinated debt securities (the “Subordinated Debt Securities”). The Senior Debt Securities will be issued under an Indenture (the “Senior Indenture”) between us and a trustee that will be identified in a prospectus supplement (the “Senior Trustee”). The Senior Debt Securities will be our direct, unsecured obligations and will rank equally with all of our outstanding unsecured senior indebtedness. The Subordinated Debt Securities will be issued under a separate indenture (the “Subordinated Indenture”) between us and a trustee that will be identified in a prospectus supplement (the “Subordinated Trustee”), which may be the same as the Senior Trustee. The Subordinated Debt Securities will be our direct, unsecured obligations and, unless otherwise specified in the prospectus supplement relating to a particular series of Subordinated Debt Securities offered by such prospectus supplement, will be subject to the subordination provisions set forth under the heading “Subordination of the Subordinated Debt Securities” below. The Senior Indenture and the Subordinated Indenture are together called the “Indentures” and the Senior Trustee and the Subordinated Trustee are together called the “Trustee.”

The following summary of certain provisions of the Indentures is not complete. You should refer to the form of each Indenture, copies of which will be filed as exhibits to the registration statement of which this prospectus is a part.

The following section describes certain general terms and provisions of the Debt Securities. The Debt Securities may be issued from time to time in one or more series. The particular terms of each series of Debt Securities offered by any prospectus supplement will be described in that prospectus supplement.

General

The Indentures do not limit the aggregate principal amount of Debt Securities that we may issue. Each Indenture provides that Debt Securities of any series may be issued under it up to the aggregate principal amount authorized from time to time by us and may be denominated in any currency or currency unit that we designate. We will determine the terms and conditions of each series of Debt Securities, including the maturity, principal and interest, but those terms must be consistent with the applicable Indenture. Unless set forth in the applicable prospectus supplement, neither the Indentures nor the Debt Securities will limit or otherwise restrict the amount of other indebtedness that we may incur or the other securities that we may issue.

The prospectus supplement relating to each series of Debt Securities being offered will specify the particular terms of those Debt Securities. The terms may include:

• the title of the Debt Securities and whether they are Senior Debt Securities or Subordinated Debt Securities;

• any limit on the aggregate principal amount of the Debt Securities;

• the priority of payment of the Debt Securities, including any subordination provisions;

• the price or prices (which may be expressed as a percentage of the aggregate principal amount thereof) at which the Debt Securities will be issued;

• the date or dates on which the principal and premium, if any, of the Debt Securities are payable;

• the interest rate or rates (which may be fixed or variable) of the Debt Securities, if any;

• the interest payment date or dates, if any, or the method or methods by which such dates may be determined, if any, the date or dates on which payment of interest, if any, will commence, the date or dates from which interest will accrue and the regular record dates for such interest payment dates;

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• the extent to which any of the Debt Securities will be issuable in temporary or permanent global form, or the manner in which any interest payable on a temporary or permanent Global Security (as defined herein) will be paid;

• each office or agency where, subject to the terms of the applicable Indenture, the Debt Securities may be presented for registration of transfer or exchange;

• the place or places where, subject to the terms of the applicable Indenture, the principal (and premium, if any) and interest, if any, on the Debt Securities will be payable;

• the terms and conditions on which we may redeem any Debt Securities, if at all;

• any obligation to redeem or purchase any Debt Securities and the terms and conditions on which we must do so;

• the denomination or denominations in which the Debt Securities will be issuable if other than $1,000 and integral multiples thereof;

• the currency, currencies or units based on or related to currencies for which the Debt Securities may be purchased and the currency, currencies or currency units in which the principal of, premium, if any, and any interest on such Debt Securities may be payable;

• whether the Debt Securities will be convertible into shares of our common stock or other securities or property, and, if so, the terms of such conversion;

• any index used to determine the amount of payments of principal of, premium, if any, and interest on the Debt Securities;

• the payment of any additional amounts with respect to the Debt Securities;

• whether any of the Debt Securities will be issued as Original Issue Discount Securities (as defined below) and the terms and provisions relating to these securities;

• information with respect to book-entry procedures relating to Global Securities, if any;

• if applicable, that the Debt Securities are defeasible;

• any additional covenants or events of default not set forth in the applicable Indenture or changes in the covenants or events of default set forth in the applicable Indenture; and

• any other terms of the Debt Securities not inconsistent with the provisions of the applicable Indenture.

Debt Securities may be issued as original issue discount Debt Securities (bearing no interest or interest at a rate that at the time of issuance is below market rates) (“Original Issue Discount Securities”), to be sold at a substantial discount below their stated principal amount. There may not be any periodic payments of interest on Original Issue Discount Securities. In the event of an acceleration of the maturity of any Original Issue Discount Security, the amount payable to the holder of such Original Issue Discount Security upon such acceleration will be set forth in the prospectus supplement and determined in accordance with the terms of such security and the Indenture, but will be an amount less than the amount payable at the maturity of the principal of such Original Issue Discount Security. The federal income tax considerations with respect to Original Issue Discount Securities will be explained in the prospectus supplement we prepare for the Original Issue Discount Securities.

Conversion and Exchange Rights

The prospectus supplement will describe, if applicable, the terms on which you may convert Debt Securities into or exchange them for our common stock or other securities or property. The conversion or exchange may be mandatory or may be at your option. We will describe how the number of shares of our common stock or other securities or property to be received upon conversion or exchange would be calculated.

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Form, Exchange and Transfer

We will issue Debt Securities only in fully registered form, without coupons, and, unless otherwise specified in the prospectus supplement, only in denominations of $1,000 and integral multiples thereof.

The holder of a Debt Security may elect, subject to the terms of the Indentures and the limitations applicable to Global Securities, to exchange them for other Debt Securities of the same series of any authorized denomination and of a like tenor and aggregate principal amount.

Holders of Debt Securities may present them for exchange as provided above or for registration of transfer, duly endorsed or with the form of transfer endorsed thereon duly executed, at the office of the transfer agent we designate for the purpose. We will not impose a service charge for any registration of transfer or exchange of Debt Securities, but we may require a payment sufficient to cover any tax or other governmental charge payable in connection with the transfer exchange. We will name the transfer agent in the prospectus supplement. We may designate additional transfer agents or rescind the designation of any transfer agent or approve a change in the office through which any transfer agent acts, but we must maintain a transfer agent in each place of payment for the Debt Securities.

If we redeem the Debt Securities, we will not be required to issue, register the transfer of or exchange any Debt Security during a specified period prior to mailing a notice of redemption. We are not required to register the transfer of or exchange any Debt Security selected for redemption, except the unredeemed portion of the Debt Security being redeemed.

Payment and Paying Agents

Unless otherwise stated in the prospectus supplement, we will pay principal and any premium or interest on a Debt Security to the person in whose name the Debt Security is registered at the close of business on the regular record date for such interest.

Unless otherwise stated in the prospectus supplement, we will pay principal and any premium or interest on the Debt Securities at the office of our designated paying agent, except we may pay interest by check mailed to the address of the person entitled to the payment. Unless we state otherwise in the prospectus supplement, the corporate trust office of the Trustee will be the paying agent for the Debt Securities.

Any other paying agents we designate for the Debt Securities of a particular series will be named in the prospectus supplement. We may designate additional paying agents, rescind the designation of any paying agent or approve a change in the office through which any paying agent acts, but we must maintain a paying agent in each place of payment for the Debt Securities.

The paying agent will return to us all money we pay to it for the payment of the principal, premium or interest on any Debt Security that remains unclaimed for a specified period. The holder thereafter may look only to us for payment.

Global Securities

The Debt Securities of any series may be represented by one or more global securities (each, a “Global Security” and, together, the “Global Securities”) that will have an aggregate principal amount equal to that of the Debt Securities of that series. Each Global Security will be registered in the name of a depository identified in the prospectus supplement. We will deposit the Global Security with the depository or a custodian, and the Global Security will bear a legend regarding the restrictions on exchanges and registration of transfer.

No Global Security may be exchanged in whole or in part for Debt Securities registered, and no transfer of a Global Security in whole or in part may be registered, in the name of any person other than the depository or any nominee of the depository unless (1) the depository has notified us that it is unwilling or unable to continue as

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depository or (2) an event of default occurs and continues with respect to the Debt Securities. The depository will determine how all securities issued in exchange for a Global Security will be registered.

As long as the depository or its nominee is the registered holder of a Global Security, the depository or the nominee will be considered the sole owner and holder of the Global Security and the underlying Debt Securities. Except as stated above, owners of beneficial interests in a Global Security will not be entitled to have the Global Security or any Debt Security registered in their names, will not receive physical delivery of certificated Debt Securities and will not be considered to be the owners or holders of the Global Security or underlying Debt Securities. We will make all payments of principal, premium and interest on a Global Security to the depository or its nominee. The laws of some jurisdictions require that certain purchasers of securities take physical delivery of such securities in definitive form. These laws may prevent you from transferring your beneficial interests in a Global Security.

Only institutions that have accounts with the depository or its nominee and persons that hold beneficial interests through the depository or its nominee may own beneficial interests in a Global Security. The depository will credit, on its book-entry registration and transfer system, the respective principal amounts of Debt Securities represented by the Global Security to the accounts of its participants. Ownership of beneficial interests in a Global Security will be shown only on, and the transfer of those ownership interests will be effected only through, records maintained by the depository or any such participant.

The policies and procedures of the depository may govern payments, transfers, exchanges and other matters relating to beneficial interests in a Global Security. We and the Trustee assume no responsibility or liability for any aspect of the depository’s or any participant’s records relating to, or for payments made on account of, beneficial interests in a Global Security. The specific terms of the depository arrangement with respect to any series of Debt Securities will be described in the applicable prospectus supplement.

Consolidation, Merger and Sale of Assets

Each Indenture provides that we may, without the consent of the holders of any of the Debt Securities outstanding under the applicable Indenture, consolidate with, merge into or transfer our assets substantially as an entirety to any person, provided that:

• any successor assumes our obligations on the applicable Debt Securities and under the applicable Indenture;

• after giving effect to the transaction, there is no Default or Event of Default (as defined in the applicable Indenture) that is continuing; and

• certain other conditions under the applicable Indenture are met.

Accordingly, such consolidation, merger or transfer of assets substantially as an entirety, which meets the conditions described above, would not create any event of default which would entitle holders of the Debt Securities, or the Trustee on their behalf, to take any of the actions described below under “Events of Default.”

Leveraged and Other Transactions

Unless otherwise specified in the applicable prospectus supplement, the Indentures and the Debt Securities will not contain, among other things, provisions that would protect holders of the Debt Securities in the event of a highly leveraged or other transaction involving us that could adversely affect the holders of Debt Securities.

Modification of the Indentures; Waiver

Each Indenture provides that, with the consent of the holders of not less than a majority in aggregate principal amount of the outstanding Debt Securities of each affected series, modifications and alterations of such

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Indenture may be made that affect the rights of the holders of such Debt Securities. However, no such modification or alteration may be made without the consent of the holder of each Debt Security so affected which would, among other things:

• change the maturity of the principal of, or of any installment of interest (or premium, if any) on, any Debt Security issued pursuant to such Indenture;

• change the principal amount thereof, premium thereon, if any, or interest thereon;

• change the method of calculation of interest or the currency of payment of principal or interest (or premium, if any) thereon;

• reduce the minimum rate of interest thereon;

• impair the right to bring suit for the enforcement of any such payment on or with respect to any such Debt Security;

• reduce the amount of principal of an Original Issue Discount Security that would be due and payable upon an acceleration of the maturity thereof;

• reduce the above-stated percentage in principal amount of outstanding Debt Securities of any series required to modify or alter such Indenture;

• in the case of Subordinated Debt Securities, modify the subordination provisions in a manner materially adverse to their holders;

• in the case of Debt Securities that are convertible or exchangeable into our other securities, adversely affect the right of holders to convert or exchange any of the Debt Securities;

• reduce the percentage in principal amount of outstanding Debt Securities of any series necessary for waiver of compliance with certain provisions of the Indentures or for waiver of certain defaults;

• modify provisions with respect to modification and waiver; or

• change our obligation to maintain an office or agency as required by the applicable Indenture.

The holders of a majority in aggregate principal amount of the outstanding Debt Securities of any series may waive, on behalf of the holders of all Debt Securities of that series, our compliance with certain restrictive provisions of the Indentures. Prior to the acceleration of the maturity of the Debt Securities of any series outstanding under the Indentures, the holders of a majority in aggregate principal amount of the outstanding Debt Securities of any series may waive any past default under the Indenture with respect to Debt Securities of that series, except a default (1) in the payment of principal, premium or interest on any Debt Security of that series or (2) in respect of a covenant or provision of the Indenture that cannot be amended without each holder’s consent.

Except in certain limited circumstances, we may set any day as a record date for the purpose of determining the holders of outstanding Debt Securities of any series entitled to give or take any direction, notice, consent, waiver or other action under the Indentures. In certain limited circumstances, the Trustee may set a record date for action by holders. To be effective, the action must be taken by holders of the requisite principal amount of such Debt Securities within a specified period following the record date.

Events Of Default

An Event of Default with respect to the Debt Securities of any series is defined in the applicable Indenture as:

• default in the payment of principal of or premium, if any, on any Debt Security of that series when due, whether or not, in the case of Subordinated Debt Securities, such payment is prohibited by the Subordinated Indenture;

• default in the payment of interest on any Debt Security of that series when due, which continues for 30 days, whether or not, in the case of Subordinated Debt Securities, such payment is prohibited by the Subordinated Indenture;

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• failure to deposit any sinking fund payment, when due, in respect of any Debt Security of that series, whether or not, in the case of Subordinated Debt Securities, such payment is prohibited by the subordination provisions of the Subordinated Indenture;

• default in the performance by us of any of our other covenants in the applicable Indenture with respect to the Debt Securities of such series, which continues for 90 days after written notice by the Trustee or the holders of at least 25% in aggregate principal amount of the Debt Securities of that series;

• certain events of bankruptcy, insolvency or reorganization affecting us; and

• any other event that may be specified in a prospectus supplement with respect to any series of Debt Securities.

If an Event of Default (other than an Event of Default relating to events of bankruptcy, insolvency or reorganization) with respect to any series of Debt Securities occurs and is continuing, either the Trustee or the holders of at least 25% in aggregate principal amount of the Debt Securities of such series outstanding may declare the principal amount (or if such Debt Securities are Original Issue Discount Securities, such portion of the principal amount as may be specified in the terms of that series) of all Debt Securities of that series to be immediately due and payable. If an Event of Default relating to events of bankruptcy, insolvency or reorganization with respect to the Debt Securities of any series at the time outstanding shall occur, the principal amount of all the Debt Securities of that series (or, in the case of any such Original Issue Discount Security, such specified amount) will automatically, and without any action by the applicable Trustee or any holder, become immediately due and payable. After any such acceleration, but before a judgment or decree based on acceleration, the holders of a majority in principal amount of the outstanding Debt Securities of that series may, under certain circumstances, rescind and annul such acceleration if all Events of Default, other than the non-payment of accelerated principal (or other specified amount), have been cured or waived as provided in the applicable Indenture. For information as to waiver of defaults, see “Modification of the Indentures; Waiver.”

If an Event of Default occurs and is continuing, the Trustee may, in its discretion, and at the written request of holders of not less than a majority in aggregate principal amount of the Debt Securities of any series, and upon reasonable indemnity against the costs, expenses and liabilities to be incurred in compliance with such request and subject to certain other conditions set forth in the applicable Indenture will, proceed to protect the rights of the holders of all the Debt Securities of such series.

The Indentures provide that upon the occurrence of an Event of Default relating to payments of principal of, premium, if any, or interest on any Debt Security, we will, upon demand of the Trustee, pay to it, for the benefit of the holder of any such Debt Security, the whole amount then due and payable on such Debt Securities for principal, premium, if any, and interest. The Indentures further provide that that if we fail to pay such amount upon such demand, the Trustee may, among other things, institute a judicial proceeding for the collection of the amount due.

No holder of a Debt Security of any series may institute any proceeding with respect to the Indentures, or for the appointment of a receiver or a trustee, or for other remedy, unless (1) the holder has previously given the Trustee written notice of a continuing event of default, (2) the holders of at least 25% in aggregate principal amount of the outstanding Debt Securities of that series have made a written request, and the holders have offered reasonable indemnity to the Trustee to institute the proceeding, and (3) the Trustee has failed to institute the proceeding, and has not received a direction inconsistent with the request within 60 days of such notice. The Indentures also provide that, notwithstanding any other provision of the applicable Indenture, the holder of any Debt Security of any series will have the right to institute suit for the enforcement of any payment of principal of, premium, if any, and interest on such Debt Securities when due and that such right will not be impaired without the consent of such holder.

We are required to file annually with the applicable Trustee a written statement as to the existence or non-existence of defaults under the Indentures or the Debt Securities.

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Subordination of the Subordinated Debt Securities

The Subordinated Debt Securities will be our direct, unsecured obligations and, unless otherwise specified in the prospectus supplement relating to a particular series of Subordinated Debt Securities offered by such prospectus supplement, will be subject to the subordination provisions described in this section. Upon any distribution of our assets due to any dissolution, winding up, liquidation or reorganization, the payment of the principal of, premium, if any, and interest on the Subordinated Debt Securities is to be subordinated in right of payment to all Senior Indebtedness. In certain events of bankruptcy or insolvency, the payment of the principal of and interest on the Subordinated Debt Securities will, to the extent provided in the Subordinated Indenture, also be effectively subordinated in right of payment to all General Obligations (as defined below).

Upon any distribution of our assets due to any dissolution, winding up, liquidation or reorganization, the holders of Senior Indebtedness will first be entitled to receive payment in full of all amounts due or to become due before the holders of the Subordinated Debt Securities will be entitled to receive any payment in respect of the Subordinated Debt Securities. If upon any such payment or distribution of assets, after giving effect to such subordination provisions in favor of the holders of Senior Indebtedness, (i) there remain any amounts of cash, property or securities available for payment or distribution in respect of the Subordinated Debt Securities (“Excess Proceeds”) and (ii) if, at such time, any creditors in respect of General Obligations have not received payment in full of all amounts due or to become due on or in respect of such General Obligations, then such Excess Proceeds will first be applied to pay or provide for the payment in full of such General Obligations before any payment or distribution may be made in respect of the Subordinated Debt Securities.

In addition, no payment may be made on the Subordinated Debt Securities, or in respect of any redemption, retirement, purchase or other acquisition of any of the Subordinated Debt Securities, at any time in the event:

• there is a default in the payment of the principal of, premium, if any, interest on or otherwise in respect of any Senior Indebtedness; or

• any event of default with respect to any Senior Indebtedness has occurred and is continuing or would occur as a result of such payment on the Subordinated Debt Securities or any redemption, retirement, purchase or other acquisition of any of the Subordinated Debt Securities, permitting the holders of such Senior Indebtedness to accelerate the maturity thereof.

Except as described above, our obligation to make payments of the principal of, premium, if any, or interest on the Subordinated Debt Securities will not be affected.

By reason of the subordination in favor of the holders of Senior Indebtedness, in the event of a distribution of assets upon any dissolution, winding up, liquidation or reorganization, our creditors who are not holders of Senior Indebtedness or the Subordinated Debt Securities may recover less, proportionately, than holders of Senior Indebtedness and may recover more, proportionately, than holders of the Subordinated Debt Securities.

Subject to payment in full of all Senior Indebtedness, the holders of Subordinated Debt Securities will be subrogated to the rights of the holders of Senior Indebtedness to receive payments or distributions of cash, property or our securities applicable to Senior Indebtedness. Subject to payment in full of all General Obligations, the holders of the Subordinated Debt Securities will be subrogated to the rights of the creditors in respect of General Obligations to receive payments or distributions of cash, property or our securities applicable to such creditors in respect of General Obligations.

“Senior Indebtedness” for purposes of the Subordinated Indenture is the principal of, premium, if any, and interest on:

• all of our indebtedness for money borrowed (other than (i) the Subordinated Debt Securities and (ii) the Junior Subordinated Indebtedness (as defined below)) whether outstanding on the date of execution of the Subordinated Indenture or created, assumed or incurred after that date, except such indebtedness as

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is by its terms expressly stated to be not superior in right of payment to the Subordinated Debt Securities or to rank equally with the Subordinated Debt Securities; and

• any deferrals, renewals or extensions of any such Senior Indebtedness.

The term “indebtedness for money borrowed” as used in this prospectus includes, without limitation, any obligation of, or any obligation guaranteed by us for the repayment of borrowed money, whether or not evidenced by bonds, debentures, notes or other written instruments, and any deferred obligation for the payment of the purchase price of property or assets.

The Subordinated Indenture does not limit our issuance of additional Senior Indebtedness. The Subordinated Debt Securities will rank senior in right of payment to our Junior Subordinated Indebtedness upon any distribution of our assets due to any dissolution, winding up, liquidation or reorganization, to the extent provided in the instruments creating our Junior Subordinated Indebtedness.

“Junior Subordinated Indebtedness” is the principal of, premium, if any, and interest on:

• all of our indebtedness for money borrowed whether outstanding on the date of the execution of the Subordinated Indenture or created, assumed or incurred after that date that is by its terms subordinated to the Subordinated Debt Securities; and

• any deferrals, renewals or extensions of any of such Junior Subordinated Indebtedness.

Unless otherwise specified in the prospectus supplement relating to a particular series of Subordinated Debt Securities offered thereby, the term “General Obligations” means all obligations to make payment on account of claims in respect of derivative products such as interest and foreign exchange rate contracts, commodity contracts and similar arrangements, other than:

• obligations on account of Senior Indebtedness;

• obligations on account of indebtedness for money borrowed ranking equal with or subordinate to the Subordinated Debt Securities; and

• obligations which by their terms are expressly stated not to be senior in right of payment to the Subordinated Debt Securities or to rank equally with the Subordinated Debt Securities.

Unless otherwise specified in the prospectus supplement relating to any series of Subordinated Debt Securities, payment of principal of the Subordinated Debt Securities may be accelerated only in case of the bankruptcy, insolvency or reorganization of our company.

Defeasance and Covenant Defeasance

To the extent stated in the prospectus supplement, we may elect to apply the provisions relating to defeasance and discharge of indebtedness, or to defeasance of certain restrictive covenants in the Indentures, to the Debt Securities of any series.

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DESCRIPTION OF CAPITAL STOCK

The following summary of provisions of our capital stock describes all material provisions of, but does not purport to be complete and is subject to, and qualified in its entirety by, our restated certificate of incorporation and our amended and restated by-laws. Our restated certificate of incorporation and amended and restated bylaws are included as exhibits to the registration statement of which this prospectus forms a part. You should read our certificate of incorporation and our by-laws for additional information before you make an investment decision. See “Where You Can Find More Information.”

General

As of September 27, 2003, our authorized common stock was 250,000,000 shares, of which 39,912,982 shares were issued and outstanding and held of record by 2,257 stockholders. The restated certificate and amended and restated by-laws contain provisions that are intended to enhance the likelihood of continuity and stability in the composition of our board of directors. These provisions may have the effect of delaying, deferring or preventing a future takeover or change in control of Mykrolis unless such takeover or change in control is approved by our board of directors.

Common Stock

The issued and outstanding shares of common stock are, and the shares of common stock to be registered hereunder will be, validly issued, fully paid and nonassessable. Subject to the prior rights of the holders of any series of preferred stock, the holders of outstanding shares of common stock are entitled to receive dividends out of assets legally available therefor at such time and in such amounts as the board of directors may from time to time determine. Please see “Dividend Policy.” The shares of common stock are not convertible and the holders thereof have no preemptive or subscription rights to purchase any of our securities. Upon liquidation, dissolution or winding up of Mykrolis, the holders of common stock are entitled to receive pro rata our assets which are legally available for distribution, after payment of all debts and other liabilities and subject to the prior rights of any holders of any series of preferred stock then outstanding. Each outstanding share of common stock is entitled to one vote on all matters submitted to a vote of stockholders. There is no cumulative voting. Except as otherwise required by law or the restated certificate, the holders of common stock vote together will all other shares of capital stock as a single class on all matters submitted to a vote of stockholders, including votes to increase the number of authorized shares of common stock. As a result, if we issue shares of capital stock other than common stock, including shares of preferred stock with super voting rights, holders of those shares could influence stockholder votes to increase the authorized shares of our common stock, which could dilute our common stockholders’ ownership of Mykrolis and reduce their control over matters submitted to our stockholders.

Our common stock is listed on the New York Stock Exchange under the symbol “MYK.”

Preferred Stock

Our board of directors may, without further action by our stockholders, from time to time, direct the issuance of shares of preferred stock in series and may, at the time of issuance, determine the rights, preferences and limitations of each series. Satisfaction of any dividend preferences of outstanding shares of preferred stock would reduce the amount of funds available for the payment of dividends on shares of common stock. Holders of shares of preferred stock may be entitled to receive a preference payment in the event of any liquidation, dissolution or winding-up of Mykrolis before any payment is made to the holders of shares of common stock. The issuance of shares of preferred stock may render more difficult or tend to discourage a merger, tender offer or proxy contest, the assumption of control by a holder of a large block of our securities or the removal of incumbent management. Upon the affirmative vote of a majority of the total number of directors then in office, the board of directors of Mykrolis, without stockholder approval, may issue shares of preferred stock with voting and conversion rights which could adversely affect the holders of shares of common stock. As of September 27, 2003, there were no shares of preferred stock outstanding.

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We have no current intention to issue any of our authorized but unissued shares of preferred stock. However, the issuance of any shares of preferred stock in the future could adversely affect the rights of the holders of common stock.

Shareholder Rights Plan

In March of 2001, our board of directors adopted a shareholder rights plan pursuant to which a dividend was declared on November 29, 2001 to all of our shareholders of record on December 31, 2001 a common stock purchase right, entitling the holder to purchase one additional share of our common stock for each share of our common stock owned on that record date, at a price of $130.00 for each share. The shareholder rights plan is designed to protect our shareholders from attempts by others to acquire our company on terms or by using tactics that could deny all shareholders the opportunity to realize the full value of their investment. The common stock purchase rights are attached to the shares of our common stock until certain triggering events occur. These purchase rights authorize the holders to purchase shares of our common stock at a 50% discount from market value upon the occurrence of specified triggering events, including, unless approved by our board of directors, an acquisition by a person or group of specified levels of beneficial ownership of our common stock or a tender offer for our common stock. The common stock purchase rights are redeemable by us for $0.01 and will expire in March of 2011. One of the events which will trigger the common stock purchase rights is the acquisition, or commencement of a tender offer, by a person, other than our company or any of our subsidiaries or employee benefit plans, of 15% or more of the outstanding shares of our common stock. Any such acquiring person may not exercise a common stock purchase right.

A copy of the common stock rights agreement is filed as an exhibit to the registration statement of which this prospectus forms a part and is incorporated herein by reference.

Other Provisions of the Restated Certificate of Incorporation and Amended and Restated By-laws

A number of provisions of our restated certificate of incorporation and amended and restated by-laws concern matters of corporate governance and the rights of stockholders. These provisions include the following:

Classified Board. Our restated certificate of incorporation provides for the board to be divided into three classes, as nearly equal in number as possible, serving staggered terms. Approximately one-third of the board will be elected each year. Under the Delaware General Corporation Law, directors serving on a classified board can only be removed for cause. The provision for a classified board could prevent a party who acquires control of a majority of the outstanding voting stock from obtaining control of the board until the second annual stockholders meeting following the date the acquiror obtains the controlling stock interest. The classified board provision could have the effect of discouraging a potential acquiror from making a tender offer or otherwise attempting to obtain control of Mykrolis and could increase the likelihood that incumbent directors will retain their positions.

Elimination of Stockholder Action Through Written Consents. Our restated certificate of incorporation provides that stockholder action can be taken only at an annual or special meeting of stockholders and cannot be taken by written consent in lieu of a meeting.

Elimination of the Ability to Call Special Meetings. Our restated certificate of incorporation and amended and restated by-laws provide that, except as otherwise required by law, special meetings of the stockholders can only be called pursuant to a resolution adopted by a majority of the board of directors or by our chairman or chief executive officer. Stockholders are not permitted to call a special meeting or to require the board to call a special meeting.

Advanced Notice Procedures For Stockholder Proposals. Our amended and restated by-laws establish an advance notice procedure for stockholder proposals to be brought before an annual meeting of our stockholders, including proposed nominations of persons for election to the board. Stockholders at our annual meeting may

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only consider proposals or nominations specified in the notice of meeting or brought before the meeting by or at the direction of the board or by a stockholder who was a stockholder of record on the record date for the meeting, who is entitled to vote at the meeting and who has given to our secretary timely written notice, in proper form, of the stockholder’s intention to bring that business before the meeting. Although our by-laws do not give the board the power to approve or disapprove stockholder nominations of candidates or proposals regarding other business to be conducted at a special or annual meeting, our by-laws may have the effect of precluding the conduct of business at a meeting if the proper procedures are not followed or may discourage or deter a potential acquiror from conducting a solicitation of proxies to elect its own slate of directors or otherwise attempting to obtain control of Mykrolis.

Amendments to the Restated Certificate and Amended and Restated By-laws. Our restated certificate of incorporation and amended and restated by-laws provide that the affirmative vote of holders of at least 75% of the total votes eligible to be cast in the election of directors is required to amend, alter, change or repeal provisions relating to by-law amendments, procedures for electing and removing directors, calling special meetings of stockholders and restrictions on stockholder actions by written consent. This requirement of a super-majority vote to approve amendments to our restated certificate and by-laws could enable a minority of our stockholders to exercise veto power over any such amendments.

No Cumulative Voting. Our restated certificate of incorporation and amended and restated by-laws do not provide for cumulative voting in the election of directors.

Undesignated Preferred Stock. The authorization of undesignated preferred stock makes it possible for our board of directors to issue preferred stock with voting or other rights or preferences that could impede the success of any attempt to change control of us. These and other provisions may have the effect of delaying or deterring hostile takeovers or changes in control or management of us.

Provisions of Delaware Law Governing Business Combinations

We are subject to Section 203 of the Delaware General Corporation Law, which is the “business combination” provisions of the Delaware General Corporation Law. In general, such provisions prohibit a publicly-held Delaware corporation from engaging in various “business combination” transactions with any “interested stockholder” for a period of three years after the date of the transaction in which the person became an “interested stockholder,” unless:

• the transaction is approved by the board of directors prior to the date the “interested stockholder” obtained such status;

• upon consummation of the transaction which resulted in the stockholder becoming an “interested stockholder,” the “interested stockholder” owned at least 85% of the voting stock of the corporation outstanding at the time the transaction commenced, excluding for purposes of determining the number of shares outstanding those shares owned by (a) persons who are directors and also officers, and (b) employee stock plans in which employee participants do not have the right to determine confidentially whether shares held subject to the plan will be tendered in a tender or exchange offer; or

• on or subsequent to such date the “business combination” is approved by the board of directors and authorized at an annual or special meeting of stockholders by the affirmative vote of at least 66 ²/3% of the outstanding voting stock which is not owned by the “interested stockholder.”

A “business combination” is defined to include mergers, asset sales and other transactions resulting in financial benefit to a stockholder. In general, an “interested stockholder” is a person who, together with affiliates and associates, owns 15% or more of a corporation’s voting stock or within three years did own 15% or more of a corporation’s voting stock. The statute could prohibit or delay mergers or other takeover or change of control attempts with respect to Mykrolis and, accordingly, may discourage attempts to acquire Mykrolis.

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Section 203 of the Delaware General Corporation Law may make it more difficult for an “interested stockholder” to effect various business combinations with us for a three-year period. The existence of this provision would be expected to have an anti-takeover effect with respect to transactions not approved in advance by the board of directors, including discouraging attempts that might result in a premium over the market price for the shares of common stock held by stockholders. A Delaware corporation may “opt out” of Section 203 with an express provision in its original certificate of incorporation or any amendment thereto. Our certificate of incorporation does not contain any such exclusion.

Limitations on Liability and Indemnification of Officers and Directors

Our restated certificate of incorporation limits the liability of directors to the fullest extent permitted by Delaware law. In addition, our restated certificate of incorporation provides that we will indemnify our directors and officers to the fullest extent permitted by such law. We have entered into indemnification agreements with our current directors and executive officers and expect to enter into a similar agreement with any new directors or executive officers. We maintain in effect directors’ and officers’ insurance coverage against specified liabilities.

Transfer Agent and Registrar

The transfer agent and registrar for our common stock is Equiserve Trust Company N.A.

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PLAN OF DISTRIBUTION

General

We may sell the securities offered by this prospectus in one or more of the following ways:

• through underwriters;

• through agents; or

• directly to a limited number of institutional purchasers or to a single purchaser.

The prospectus supplement for the securities we sell will describe that offering, including:

• the name or names of any underwriters or agents;

• the purchase price and the proceeds to us from that sale;

• any over-allotment options under which underwriters may purchase additional securities from us;

• any agency fees or underwriting discounts and other items constituting agents’ or underwriters’ compensation; and

• any initial public offering price and any discounts or concessions allowed or reallowed or paid to dealers.

Underwriters

If underwriters are used in the sale, we will execute an underwriting agreement with those underwriters relating to the securities that we will offer. Unless otherwise provided in the applicable prospectus supplement, the obligations of the underwriters to purchase these securities will be subject to conditions. The underwriters will be obligated to purchase all of these securities if any are purchased.

The securities subject to the underwriting agreement will be acquired by the underwriters for their own account and may be resold by them from time to time in one or more transactions, including negotiated transactions, at a fixed public offering price or at varying prices determined at the time of sale. Underwriters may be deemed to have received compensation from us in the form of underwriting discounts or commissions and may also receive commissions from the purchasers of these securities for whom they may act as agent. Underwriters may sell these securities to or through dealers. These dealers may receive compensation in the form of discounts, concessions or commissions from the underwriters or commissions from the purchasers for whom they may act as agent. Any initial public offering price and any discounts or concessions allowed or reallowed or paid to dealers may be changed from time to time.

We will identify in the applicable prospectus supplement any underwriters, dealers or agents and will describe their compensation. Underwriters, dealers and agents may engage in transactions with or perform services for us or our subsidiaries in the ordinary course of business.

We also may sell the securities in connection with a remarketing upon their purchase, in connection with a redemption or repayment, by a remarketing firm acting as principal for its own account or as our agent. Remarketing firms may be deemed to be underwriters in connection with the securities that they remarket.

We may authorize underwriters to solicit offers by institutions to purchase the securities subject to the underwriting or terms agreement from us at the public offering price stated in the prospectus supplement under delayed delivery contracts providing for payment and delivery on a specified date in the future. If we sell securities under these delayed delivery contracts, the prospectus supplement will state that as well as the conditions to which these delayed delivery contracts will be subject and the commissions payable for that solicitation.

In connection with an offering, an underwriter may purchase and sell securities in the open market. These transactions may include short sales, stabilizing transactions and purchases to cover positions created by short sales. Shorts sales involve the sale by the underwriters of a greater number of securities than they are required to purchase in the offering. “Covered” short sales are sales made in an amount not greater than the underwriters’ option to purchase additional securities from the Company in the offering, if any. If the underwriters have an

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over-allotment option to purchase additional securities from the Company, the underwriters may close out any covered short position by either exercising their over-allotment option or purchasing securities in the open market. In determining the source of securities to close out the covered short position, the underwriters may consider, among other things, the price of securities available for purchase in the open market as compared to the price at which they may purchase securities through the over-allotment option. “Naked” short sales are any sales in excess of such option or where the underwriters do not have an over-allotment option. The underwriters must close out any naked short position by purchasing securities in the open market. A naked short position is more likely to be created if the underwriters are concerned that there may be downward pressure on the price of the securities in the open market after pricing that could adversely affect investors who purchase in the offering.

Accordingly, to cover these short sales positions or to otherwise stabilize or maintain the price of the securities, the underwriters may bid for or purchase securities in the open market and may impose penalty bids. If penalty bids are imposed, selling concessions allowed to syndicate members or other broker-dealers participating in the offering are reclaimed if securities previously distributed in the offering are repurchased, whether in connection with stabilization transactions or otherwise. The effect of these transactions may be to stabilize or maintain the market price of the securities at a level above that which might otherwise prevail in the open market. The imposition of a penalty bid may also effect the price of the securities to the extent that it discourages resale of the securities. The magnitude or effect of any stabilization or other transactions is uncertain. These transactions may be effected on the New York Stock Exchange or otherwise and, if commenced, may be discontinued at any time.

Agents

We may also sell any of the securities through agents designated by us from time to time. We will name any agent involved in the offer or sale of these securities and will list commissions payable by us to these agents in the prospectus supplement. These agents will be acting on a best efforts basis to solicit purchases for the period of their appointment, unless we state otherwise in the applicable prospectus supplement.

Direct Sales

We may sell any of the securities directly to purchasers. In this case, we will not engage underwriters or agents in the offer and sale of these securities.

Indemnification

We may indemnify underwriters, dealers or agents who participate in the distribution of the securities against certain liabilities, including liabilities under the Securities Act of 1933, and agree to contribute to payments which these underwriters, dealers or agents may be required to make.

Listing

Except as indicated in the applicable prospectus supplement, the securities are not expected to be listed on any securities exchange, except for our common stock, which is listed on the New York Stock Exchange, and any underwriters or dealers will not be obligated to make a market in these securities. We cannot predict the activity or liquidity of any trading in these securities.

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LEGAL MATTERS

Certain legal matters in connection with the securities to be offered hereby will be passed upon for Mykrolis by its counsel, Ropes & Gray LLP, Boston, Massachusetts.

EXPERTS

The financial statements of Mykrolis Corporation as of December 31, 2002 and 2001 and for each of the three years in the period ended December 31, 2002 included in this Prospectus have been so included in reliance on the report of PricewaterhouseCoopers LLP, independent accountants, given on the authority of said firm as experts in auditing and accounting.

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PART II

INFORMATION NOT REQUIRED IN PROSPECTUS

Item 14. Other Expenses of Issuance and Distribution.

The following table sets forth the various expenses payable by the registrant, other than underwriting discounts and commissions, in connection with the issuance and distribution of the securities being registered. The registrant has agreed to pay these costs and expenses.

Securities and Exchange Commission registration fee	$	16,180
National Association of Securities Dealers, Inc. filing fee		*
New York Stock Exchange listing fee		*
Trustees’ fees and expenses		*
Printing and engraving expenses		*
Legal fees and expenses		*
Accounting fees and expenses		*
Blue sky fees and expenses		*
Transfer Agent and Registrar fees		*
Miscellaneous		*
Total	$	*

* To be estimated

Item 15. Indemnification of Directors And Officers.

Mykrolis is incorporated under the laws of the State of Delaware. Section 145 (“Section 145”) of the General Corporation Law of the State of Delaware, as the same exists or may hereafter be amended (the “General Corporation Law”), inter alia, provides that a Delaware corporation may indemnify any persons who were, are or are threatened to be made, parties to any threatened, pending or completed action, suit or proceeding, whether civil, criminal, administrative or investigative (other than an action by or in the right of such corporation), by reason of the fact that such person is or was an officer, director, employee or agent of such corporation, or is or was serving at the request of such corporation as a director, officer, employee or agent of another corporation or enterprise. The indemnity may include expenses (including attorneys’ fees), judgments, fines and amounts paid in settlement actually and reasonably incurred by such person in connection with such action, suit or proceeding, provided such person acted in good faith and in a manner he reasonably believed to be in or not opposed to the corporation’s best interests and, with respect to any criminal action or proceeding, had no reasonable cause to believe that his conduct was illegal.

Section 145 further authorizes a corporation to purchase and maintain insurance on behalf of any person who is or was a director, officer, employee or agent of the corporation, or is or was serving at the request of the corporation as a director, officer, employee or agent of another corporation or enterprise, against any liability asserted against him and incurred by him in any such capacity, arising out of his status as such, whether or not the corporation would otherwise have the power to indemnify him under Section 145.

The registrant’s restated certificate of incorporation provides that the registrant’s directors shall not be liable to the registrant or its stockholders for monetary damages for breach of fiduciary duty as a director, except to the extent that the exculpation from liabilities is not permitted under the General Corporation Law as in effect at the time such liability is determined. The registrant’s restated certificate of incorporation provides that the registrant shall indemnify its directors to the full extent permitted by the laws of the State of Delaware. In addition, the registrant’s by-laws provide for indemnification of the registrant’s officers and directors to the fullest extent permitted by applicable law.

II-1

All of Mykrolis’ directors and officers are covered by insurance policies maintained by Mykrolis against specified liabilities for actions taken in their capacities as such, including liabilities under the Securities Act of 1933, as amended. In addition, Mykrolis has entered into indemnification agreements with each of its directors and executive officers that provide for indemnification and expense advancement to the fullest extent permitted under the General Corporation Law.

The master separation and distribution agreement by and between us and Millipore provides for indemnification by us of Millipore and its directors, officers and employees for some liabilities, including liabilities under the Securities Act.

Item 16. Exhibits.

1.1*	Form of Underwriting Agreement
2.1+	Form of Master Separation and Distribution Agreement (incorporated by reference to Exhibit 2.1 to the Form S-1 Registration Statement (File No. 333-57182) (the “S-1 Registration Statement”))
2.2+	Form of General Assignment and Assumption Agreement (incorporated by reference to Exhibit 2.2 to the S-1 Registration Statement)
3.1	Restated Certificate of Incorporation of Mykrolis Corporation (incorporated by reference to Exhibit 3.1 to the S-1 Registration Statement)
3.2	Amended and Restated By-laws of Mykrolis Corporation (incorporated by reference to Exhibit 3.2 to the S-1 Registration Statement)
4.1	Form of certificate representing shares of Common Stock, $.01 par value per share (incorporated by reference to Exhibit 3.3 to the S-1 Registration Statement)
4.2	Common Stock Rights Agreement (the “Rights Agreement”) dated as of November 29, 2001, between the Company and Equiserve Trust Company, N.A., as Rights Agent (incorporated by reference to Exhibit 1 to the registrant’s Current Report on Form 8-K filed with the Commission on December 10, 2001 (the “Form 8-K”))
4.3	Form of Common Stock Purchase Right Certificate (attached as Annex I to the Rights Agreement and incorporated by reference to Exhibit 2 to the Form 8-K)
4.4	Summary of Common Stock Purchase Rights (attached as Annex II to the Rights Agreement incorporated by reference to Exhibit 3 to the Form 8-K)
4.5*	Form of Senior Indenture
4.6*	Form of Subordinated Indenture
4.7*	Form of Senior Debt Security
4.8*	Form of Subordinated Debt Security
5.1	Opinion of Ropes & Gray LLP
12.1	Statement regarding computation of ratio of earnings to fixed charges
23.1	Consent of PricewaterhouseCoopers LLP
23.2	Consent of Ropes & Gray LLP (included in Exhibit 5.1)
24.1	Power of Attorney
25.1*	Statement of Eligibility and Qualification on Form T-1 of Trustee

* To be filed by amendment or as an exhibit to a report pursuant to Section 13(a), 13(c) or 15(d) of the Securities Exchange Act of 1934.

⁺ Mykrolis Corporation agrees to furnish supplementally to the Commission a copy of any omitted schedule or exhibit to such agreement upon request by the Commission.

II-2

Item 17. Undertakings.

The undersigned registrant hereby undertakes:

(1) To file, during any period in which offers or sales are being made, a post-effective amendment to this Registration Statement:

(i) to include any prospectus required by Section 10(a)(3) of the Securities Act of 1933;

(ii) to reflect in the prospectus any facts or events arising after the effective date of the Registration Statement (or the most recent post-effective amendment thereof) which, individually or in the aggregate, represent a fundamental change in the information set forth in the Registration Statement; notwithstanding the foregoing, any increase or decrease in volume of securities offered (if the total dollar value of securities offered would not exceed that which was registered) and any deviation from the low or high end of the estimated maximum offering range may be reflected in the form of prospectus filed with the Commission pursuant to Rule 424(b) if, in the aggregate, the changes in volume and price represent no more than a 20% change in the maximum aggregate offering price set forth in the “Calculation of Registration Fee” table in the effective registration statement;

(iii) to include any material information with respect to the plan of distribution not previously disclosed in the Registration Statement or any material change to such information in the Registration Statement;

provided, however, that paragraphs (i) and (ii) above do not apply if the information required to be included in a post-effective amendment by those paragraphs is contained in periodic reports filed with or furnished to the Commission by the registrant pursuant to Sections 13 or 15(d) of the Securities Exchange Act of 1934 that are incorporated by reference in the registration statement.

(2) That, for the purpose of determining any liability under the Securities Act of 1933, each such post-effective amendment shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

(3) To remove from registration by means of a post-effective amendment any of the securities being registered that remain unsold at the termination of the offering.

The undersigned registrant hereby undertakes that, for purposes of determining any liability under the Securities Act of 1933, each filing of registrant’s annual report pursuant to Section 13(a) or Section 15(d) of the Securities Exchange Act of 1934 (and, where applicable, each filing of an employee benefit plan’s annual report pursuant to Section 15(d) of the Securities Exchange Act of 1934) that is incorporated by reference in the registration statement shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

Insofar as indemnification for liabilities arising under the Securities Act of 1933 may be permitted to directors, officers and controlling persons of the Registrant pursuant to the foregoing provisions, or otherwise, the registrant has been advised that in the opinion of the Securities and Exchange Commission such indemnification is against public policy as expressed in the Act and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the registrant of expenses incurred or paid by a director, officer or controlling person of the registrant in the successful defense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securities being registered, the registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by it is against public policy as expressed in the Act and will be governed by the final adjudication of such issue.

The undersigned registrant hereby undertakes that for purposes of determining any liability under the Securities Act of 1933, the information omitted from the form of prospectus filed as part of this registration statement in reliance upon Rule 430A and contained in a form of prospectus filed by the registrant pursuant to Rule 424(b)(1) or (4) or 497(h) under the Securities Act shall be deemed to be part of this registration statement as of the time it was declared effective.

The undersigned registrant hereby undertakes to file an application for the purpose of determining the eligibility of the trustee to act under subsection (a) of Section 310 of the Trust Indenture Act in accordance with the rules and regulations prescribed by the Commission under Section 305(b)(2) of the Act.

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SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended, Mykrolis Corporation certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-3 and has duly caused this registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the Town of Billerica, Commonwealth of Massachusetts, on the 5^th day of November, 2003.

MYKROLIS CORPORATION
By:	/s/    C. WILLIAM ZADEL
	C. William Zadel Chief Executive Officer and Chairman of the Board

Pursuant to the requirements of the Securities Act of 1933, this registration statement has been signed by the following persons in the capacities and on the dates indicated:

Signature	Title	Date
/s/    C. WILLIAM ZADEL         C. William Zadel	Chairman of the Board, Chief Executive Officer (Principal Executive Officer) and Director	November 5, 2003
/s/    BERTRAND LOY         Bertrand Loy	Vice President and Chief Financial Officer (Principal Financial Officer)	November 5, 2003
/s/    ROBERT HAMMOND         Robert Hammond	Corporate Controller (Principal Accounting Officer)	November 5, 2003
* Richard A. Aurelio	Director	November 5, 2003
* Michael A. Bradley	Director	November 5, 2003
* Robert E. Caldwell	Director	November 5, 2003
* Michael P.C. Carns	Director	November 5, 2003
* Daniel W. Christman	Director	November 5, 2003
* Thomas O. Pyle	Director	November 5, 2003

The undersigned, by signing his name hereto, does sign and execute this Registration Statement on Form S-3 pursuant to the Power of Attorney executed by the above named directors and officers of the Registrant and filed with the Securities and Exchange Commission on behalf of such directors and officers.

*By: /s/    PETER W. WALCOTT         Peter W. Walcott Attorney-in-fact November 5, 2003

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EXHIBIT INDEX

Exhibit No.	Description
1.1*	Form of Underwriting Agreement
2.1+	Form of Master Separation and Distribution Agreement (incorporated by reference to Exhibit 2.1 to the Form S-1 Registration Statement (File No. 333-57182) (the “S-1 Registration Statement”))
2.2+	Form of General Assignment and Assumption Agreement (incorporated by reference to Exhibit 2.2 to the S-1 Registration Statement)
3.1	Restated Certificate of Incorporation of Mykrolis Corporation (incorporated by reference to Exhibit 3.1 to the S-1 Registration Statement)
3.2	Amended and Restated By-laws of Mykrolis Corporation (incorporated by reference to Exhibit 3.2 to the S-1 Registration Statement)
4.1	Form of certificate representing shares of Common Stock, $.01 par value per share (incorporated by reference to Exhibit 3.3 to the S-1 Registration Statement)
4.2	Common Stock Rights Agreement (the “Rights Agreement”) dated as of November 29, 2001, between the Company and Equiserve Trust Company, N.A., as Rights Agent (incorporated by reference to Exhibit 1 to the registrant’s Current Report on Form 8-K filed with the Commission on December 10, 2001 (the “Form 8-K”))
4.3	Form of Common Stock Purchase Right Certificate (attached as Annex I to the Rights Agreement and incorporated by reference to Exhibit 2 to the Form 8-K)
4.4	Summary of Common Stock Purchase Rights (attached as Annex II to the Rights Agreement incorporated by reference to Exhibit 3 to the Form 8-K)
4.5*	Form of Senior Indenture
4.6*	Form of Subordinated Indenture
4.7*	Form of Senior Debt Security
4.8*	Form of Subordinated Debt Security
5.1	Opinion of Ropes & Gray LLP
12.1	Statement regarding computation of ratio of earnings to fixed charges
23.1	Consent of PricewaterhouseCoopers LLP
23.2	Consent of Ropes & Gray LLP (included in Exhibit 5.1)
24.1	Power of Attorney
25.1*	Statement of Eligibility and Qualification on Form T-1 of Trustee

* To be filed by amendment or as an exhibit to a report pursuant to Section 13(a), 13(c) or 15(d) of the Securities Exchange Act of 1934.

⁺ Mykrolis Corporation agrees to furnish supplementally to the Commission a copy of any omitted schedule or exhibit to such agreement upon request by the Commission.