-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
Originator-Key-Asymmetric:
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 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 GNGQpSv7zUymmaNjMrz/SqCRuJ6PeQWNc3hOK7oH+cKAfoq03FLTElrkgmlDx4tf
 0Lvd7e17I6/SIFinBi8zZQ==

<SEC-DOCUMENT>0000950138-06-000723.txt : 20060919
<SEC-HEADER>0000950138-06-000723.hdr.sgml : 20060919
<ACCEPTANCE-DATETIME>20060919160732
ACCESSION NUMBER:		0000950138-06-000723
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20060915
FILED AS OF DATE:		20060919
DATE AS OF CHANGE:		20060919

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			MKS INSTRUMENTS INC
		CENTRAL INDEX KEY:			0001049502
		STANDARD INDUSTRIAL CLASSIFICATION:	INDUSTRIAL INSTRUMENTS FOR MEASUREMENT, DISPLAY, AND CONTROL [3823]
		IRS NUMBER:				042277512
		STATE OF INCORPORATION:			MA
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		90 INDUSTRIAL WAY
		CITY:			WILMINGTON
		STATE:			MA
		ZIP:			01887
		BUSINESS PHONE:		978-284-4000

	MAIL ADDRESS:	
		STREET 1:		90 INDUSTRIAL WAY
		CITY:			WILMINGTON
		STATE:			MA
		ZIP:			01887

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			EMERSON ELECTRIC CO
		CENTRAL INDEX KEY:			0000032604
		STANDARD INDUSTRIAL CLASSIFICATION:	ELECTRONIC & OTHER ELECTRICAL EQUIPMENT (NO COMPUTER EQUIP) [3600]
		STATE OF INCORPORATION:			MO
		FISCAL YEAR END:			0930

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-23621
		FILM NUMBER:		061098173

	BUSINESS ADDRESS:	
		STREET 1:		8000 W FLORISSANT AVE
		STREET 2:		P O BOX 4100
		CITY:			ST LOUIS
		STATE:			MO
		ZIP:			63136
		BUSINESS PHONE:		3145532000

	MAIL ADDRESS:	
		STREET 1:		8000 W. FLORISSANT
		STREET 2:		P.O. BOX 4100
		CITY:			ST LOUIS
		STATE:			MO
		ZIP:			63136

	FORMER NAME:	
		FORMER CONFORMED NAME:	EMERSON ELECTRIC MANUFACTUING CO
		DATE OF NAME CHANGE:	19730710
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>form4_091806ex.xml
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2006-09-15</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001049502</issuerCik>
        <issuerName>MKS INSTRUMENTS INC</issuerName>
        <issuerTradingSymbol>MKSI</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0000032604</rptOwnerCik>
            <rptOwnerName>EMERSON ELECTRIC CO</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>8000 W. FLORISSANT AVE.</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>ST LOUIS</rptOwnerCity>
            <rptOwnerState>MO</rptOwnerState>
            <rptOwnerZipCode>63136</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2006-09-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>125000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>20.5084</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3696249</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Through a subsidiary</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2006-09-18</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>25000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>20.7086</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3671249</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Through a subsidiary</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1065182</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">The sales were effected in multiple transactions, at varying prices, on September 15, 2006, as follows: 200 shares at $20.41;
 600 at $20.42; 200 at $20.43; 600 at $20.44; 806 at $20.45; 700 at $20.46; 2,280 at $20.47; 4,020 at $20.48; 17,100 at
 $20.49; 38,248 at $20.50; 24,514 at $20.51; 10,472 at $20.52; 11,078 at $20.53; 5,354 at $20.54; 2,428 at $20.55; 2,300 at
 $20.56; 3,200 at $20.57; 700 at $20.58; and 200 at $20.60.  The weighted average sales price for these transactions was
 $20.5084 per share.</footnote>
        <footnote id="F2">The sales were effected in multiple transactions, at varying prices, on September 18, 2006, as follows and as described in
 Footnote 3 below:  200 shares at $20.47; 100 at $20.49; 100 at $20.50; 800 at $20.52; 500 at $20.53; 200 at $20.54; 100 at
 $20.55; 400 at $20.56; 900 at $20.57; 1,100 at $20.58; 700 at $20.59; 1,100 at $20.60; 1,007 at $20.61; 866 at $20.62; 200
 at $20.63; 1,010 at $20.64; 900 at $20.65; 1,002 at $20.66; 300 at $20.67; 200 at $20.68; 600 at $20.69; 200 at $20.70; 200
 at $20.71; 1,800 at $20.72; 1,400 at $20.73; 800 at $20.74; and 589 at $20.75.</footnote>
        <footnote id="F3">This footnote sets forth additional detail with respect to the transactions described in Footnote 2, as follows:  200 shares
 at $20.76; 500 at $20.77; 100 at $20.78; 341 at $20.79; 200 at $20.80; 600 at $20.81; 500 at $20.82; 800 at $20.83; 500 at
 $20.84; 400 at $20.85; 200 at $20.86; 400 at $20.88; 500 at $20.89; 300 at $20.90; 300 at $20.91; 100 at $20.92; 500 at
 $20.93; 400 at $20.94; 100 at $20.95; 285 at $20.96; 300 at $20.99; and 200 at $21.00. The weighted average sales price for
 these transactions was $20.7086 per share.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Timothy G. Westman,  Assistant Secretary</signatureName>
        <signatureDate>2006-09-19</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
-----END PRIVACY-ENHANCED MESSAGE-----