EXHIBIT 4.3

                  STANDSTILL AND REGISTRATION RIGHTS AGREEMENT

THIS AGREEMENT, dated as of September 19, 2003, is between Arch Coal, Inc., a
Delaware corporation ("Arch Coal"), and ADA-ES, Inc., a Colorado corporation
("ADA-ES").

                                   WITNESSETH:

WHEREAS on the date hereof, Arch Coal is acquiring 137,741 shares (the
"Purchased Shares") of common stock of ADA-ES (the "Common Stock") and an option
(the "Option") to purchase up to 50,000 additional shares of Common Stock (the
"Option Shares") pursuant to the terms of a Securities Subscription and
Investment Agreement, dated as of July 7, 2003 (the "Subscription Agreement;"
terms capitalized and not defined herein shall have the meaning ascribed to them
in the Subscription Agreement);

WHEREAS, Arch Coal has the right, under certain circumstances, to acquire
additional shares of Common Stock pursuant to a 6% Convertible Debenture due
September 30, 2008 in the face amount of $300,000 (the "Convertible Debenture
Shares"; and together with the Purchased Shares and the Option Shares; the
"Shares")); and

WHEREAS, the execution and delivery of this Agreement by the parties is a
condition precedent to the parties' obligations under the Subscription
Agreement.

NOW, THEREFORE, in consideration of the premises and of the mutual covenants and
agreements set forth herein and for other good and valuable consideration the
receipt and sufficiency of which is hereby acknowledged, the parties, intending
to be legally bound hereby, agree as follows:

                                    ARTICLE I

                         REPRESENTATIONS AND WARRANTIES

Section 1.01 Arch Coal represents and warrants to ADA-ES as follows:

(a) Arch Coal has full legal right, power and authority to enter into and
perform this Agreement. The execution and delivery of this Agreement by Arch
Coal and the consummation by Arch Coal of the transactions contemplated hereby
have been duly authorized by all necessary corporate action on behalf of Arch
Coal. This Agreement is a valid and binding obligation of Arch Coal enforceable
against Arch Coal in accordance with its terms.

(b) Neither the execution and delivery of this Agreement by Arch Coal nor the
consummation by Arch Coal of the transactions contemplated hereby conflicts with
or constitutes a violation of or default under the charter or by-laws of Arch
Coal, any statute, law, regulation, order or decree applicable to Arch Coal, or
any contract, commitment, agreement, arrangement or restriction of any kind to
which Arch Coal is a party or by which Arch Coal is bound.

Section 1.02. ADA-ES represents and warrants to Arch Coal as follows:

(a) ADA-ES has full legal right, power and authority to enter into and perform
this Agreement. The execution and delivery of this Agreement by ADA-ES and the
consummation by ADA-ES of the transactions contemplated hereby have been duly
authorized by all necessary corporate action on behalf of ADA-ES. This Agreement
is a valid and binding obligation of ADA-ES enforceable against ADA-ES in
accordance with its terms.

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(b) Neither the execution and delivery of this Agreement by ADA-ES nor the
consummation by ADA-ES of the transactions contemplated hereby conflicts with or
constitutes a violation of or default under the charter or by-laws of ADA-ES,
any statute, law, regulation, order or decree applicable to ADA-ES, or any
contract, commitment, agreement, arrangement or restriction of any kind to which
ADA-ES is a party or by which ADA-ES is bound.

                                   ARTICLE II

                          LIMITATIONS AND RESTRICTIONS

Section 2.01. Restrictions on Sales by Arch Coal. Arch Coal agrees that until
the first anniversary of this Agreement, it will not, nor will it permit any of
its Affiliates, to sell, solicit an offer to sell, agree to sell, offer or
propose to sell (collectively "Sell"), the Shares; except as follows:

(a) Arch Coal may transfer all or a portion of the Shares to a wholly-owned
subsidiary subject to all of the terms and conditions of this Agreement,
provided that Arch Coal maintains 100% ownership and voting control of such
subsidiary, and the certificates for any securities of such subsidiary are
marked with a legend restricting the transfer of such securities and
specifically referring to this Agreement; and

(b) Arch Coal may sell its shares pursuant to a tender offer for all outstanding
shares of ADA-ES' Common Stock approved by ADA-ES' Board of Directors.

                                   ARTICLE III

                               REGISTRATION RIGHTS

Section 3.01. "Piggy-Back" Registrations. If at any time after the Distribution,
ADA-ES shall determine to register for its own account or the account of others
under the Securities Act (other than a registration demanded by Arch Coal
pursuant to Section 3.02 hereof) any of its equity securities, other than on
Form S-4 or Form S-8 or their then equivalents or otherwise relating to shares
of Common Stock to be issued in connection with any acquisition of any entity or
business or shares of Common Stock issuable in connection with stock option or
other employee benefit plans, it shall send to Arch Coal written notice of such
determination and, if within ten (10) business days after receipt of such
notice, Arch Coal shall so request in writing, ADA-ES shall use its best efforts
to include in such registration statement all or any part of the Shares then
held by Arch Coal ("Registrable Shares") and which Arch Coal requests to be
registered.

If, in connection with any offering involving an underwriting, the managing
underwriter shall impose a limitation on the number of shares of Common Stock
which may be included in the registration statement because, in its judgment,
such limitation is necessary to effect an orderly public distribution, then
ADA-ES shall be obligated to include in such registration statement only such
limited portion (which may be none) of the Registrable Shares with respect to
which Arch Coal has requested inclusion pursuant hereto as may reasonably be
determined by the managing underwriters; provided, that inclusion of any of Arch
Coal's Registrable Shares shall be subordinate to the currently existing
"piggyback" registration rights granted by ADA-ES. Any inclusion of Registrable
Shares in an offering, when the managing underwriter has so limited the number
of shares that may be included in such offering, shall be allocated as follows:
first, pro rata among the holders of registration rights granted by ADA-ES prior
to the date hereof seeking to include their shares, in proportion to the number
of shares of Common Stock (whether or not such shares are sought to be included
in such offering) held by such persons; and thereafter, to Arch Coal. ADA-ES
shall have the right to withdraw any registration initiated by it pursuant to
this Section 3.01.

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Section 3.02. Registrations on Form S-1 or Form S-2. In addition to the rights
provided Arch Coal in Section 3.01 above, if the registration of Registrable
Shares under the Securities Act can be effected on Form S-3 (or any similar form
promulgated by the Commission), then, at any time after the first anniversary of
this Agreement, upon the written request of Arch Coal, ADA-ES will use its best
efforts to effect qualification and registration under the Securities Act on
Form S-1 or Form S-2 of such portion of the Registrable Shares as Arch Coal
shall specify, up to the lesser of (i) twenty-five percent (25%) of the
Registrable Shares then held by Arch Coal, and (ii) the amount of Registrable
Shares then held by Arch Coal and permitted to be sold under Section 2.02 of
this Agreement; provided, however, ADA-ES shall not be required to effect a
registration pursuant to this Section 3.02 unless the market value of the
Registrable Shares to be sold in any such registration shall be estimated to be
at least $1,000,000 at the time of filing such registration statement, and
further provided that ADA-ES shall not be required to effect more than one (1)
registration during any twelve (12) month period pursuant to this Section 3.02
and four (4) registrations in the aggregate under this Section 3.02. No request
for registration under this Section 3.02 may be made within the one hundred and
eighty day period after the effective date of a registration statement filed by
ADA-ES or while ADA-ES is in the process of preparing a registration statement.

Section 3.03 Effectiveness. ADA-ES will use its best efforts to maintain the
effectiveness for up to 90 days (or such shorter period of time as the
underwriters need to complete the distribution of a registered offering or until
the securities are actually sold) of any registration statement pursuant to
which any of the Registrable Shares are being offered, and from time to time
will amend or supplement such registration statement and the prospectus
contained therein to the extent necessary to comply with the Securities Act and
any applicable state securities statute or regulation. ADA-ES will also provide
Arch Coal with as many copies of the prospectus contained in any such
registration statement as it may reasonably request. For a period not to exceed
ninety (90) days, ADA-ES shall not be obligated to prepare and file, or be
prevented from delaying or abandoning, a registration statement pursuant to this
Agreement at any time when ADA-ES, in its good faith judgment with advice of
counsel, reasonably believes:

(a) that the filing thereof at the time requested, or the offering of
Registrable Shares pursuant thereto, would materially and adversely affect (a) a
pending or scheduled public offering of ADA-ES's securities, (b) an acquisition,
merger, recapitalization, consolidation, reorganization or similar transaction
by or of ADA-ES, (c) pre-existing and continuing negotiations, discussions or
pending proposals with respect to any of the foregoing transactions, or (d) the
financial condition of ADA-ES in view of the disclosure of any pending or
threatened litigation, claim, assessment or governmental investigation which may
be required thereby; and

(b) that the failure to disclose any material information with respect to the
foregoing would cause a violation of the Securities Act or the Exchange Act.

Section 3.04. Indemnification of Arch Coal. In the event that ADA-ES registers
any of the Registrable Shares under the Securities Act, ADA-ES will indemnify
and hold harmless Arch Coal and each underwriter of Registrable Shares
(including their officers, directors, affiliates and partners and including any
broker or dealer through whom Registrable Shares may be sold in such
registration) and each person, if any, who controls Arch Coal or any such
underwriter within the meaning of Section 15 of the Securities Act from and
against any and all losses, claims, damages, expenses or liabilities, joint or
several, to which they or any of them become subject under the Securities Act,
applicable state securities laws or under any other statute or at common law or
otherwise, as incurred, and, except as hereinafter provided, will reimburse Arch
Coal, each such underwriter and each such controlling person, if any, for any
legal or other expenses reasonably incurred by them or any of them in connection
with investigating or defending any actions whether or not resulting in any
liability, as incurred, insofar as such losses, claims, damages, expenses,
liabilities or actions arise out of or are based upon any untrue statement or
alleged untrue statement of a material fact contained in the registration
statement, in any preliminary or amended preliminary prospectus or in the final
prospectus (or the registration statement or prospectus as from time to time
amended or supplemented by ADA-ES) or arise out of or are based upon the

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omission or alleged omission to state therein a material fact required to be
stated therein or necessary in order to make the statements therein not
misleading, or any violation by ADA-ES of any rule or regulation promulgated
under the Securities Act or any state securities laws applicable to ADA-ES and
relating to action or inaction required of ADA-ES in connection with such
registration, unless (i) such untrue statement or alleged untrue statement or
omission or alleged omission was made in such registration statement,
preliminary or amended preliminary prospectus or final prospectus in reliance
upon and in conformity with information furnished in writing to ADA-ES in
connection therewith by Arch Coal (in the case of indemnification of Arch Coal),
any such underwriter (in the case of indemnification of such underwriter) or any
such controlling person (in the case of indemnification of such controlling
person) expressly for use therein, or unless (ii) in the case of a sale directly
by Arch Coal (including a sale of Registrable Shares through any underwriter
retained by Arch Coal to engage in a distribution solely on behalf of Arch
Coal), such untrue statement or alleged untrue statement or omission or alleged
omission was contained in a preliminary prospectus and corrected in a final or
amended prospectus copies of which were delivered to Arch Coal or such
underwriter on a timely basis, and Arch Coal failed to deliver a copy of the
final or amended prospectus at or prior to the confirmation of the sale of the
Registrable Shares to the person asserting any such loss, claim, damage or
liability in any case where such delivery is required by the Securities Act.

Promptly after receipt by Arch Coal, any underwriter or any controlling person
of notice of the commencement of any action in respect of which indemnity may be
sought against ADA-ES, Arch Coal, or such underwriter or such controlling
person, as the case may be, shall notify ADA-ES in writing of the commencement
thereof (provided, that failure to so notify ADA-ES shall not relieve ADA-ES
from any liability it may have hereunder, except to the extent prejudiced by
such failure) and, subject to the provisions hereinafter stated, ADA-ES shall be
entitled to assume the defense of such action (including the employment of
counsel, who shall be counsel reasonably satisfactory to Arch Coal, such
underwriter or such controlling person, as the case may be) and the payment of
expenses insofar as such action shall relate to any alleged liability in respect
of which indemnity may be sought against ADA-ES.

Arch Coal, any such underwriter or any such controlling person shall have the
right to employ separate counsel in any such action and to participate in the
defense thereof but the fees and expenses of such counsel subsequent to any
assumption of the defense by ADA-ES shall not be at the expense of ADA-ES unless
the employment of such counsel has been specifically authorized in writing by
ADA-ES; provided, however, that, if the defendants in any such action include
both the indemnified party and the indemnifying party and the indemnified party
shall have reasonably concluded that there may be reasonable defenses available
to it which are different from or additional to those available to the
indemnifying party or if the interests of the indemnified party reasonably may
be deemed to conflict with the interests of the indemnifying party, the
indemnified party shall have the right to select a separate counsel and to
assume such legal defenses and otherwise to participate in the defense of such
action, with the expenses and fees of such separate counsel and other expenses
related to such participation to be reimbursed by the indemnifying party as
incurred. At any time, Arch Coal may select separate counsel and assume its own
legal defense with the expenses and fees of such separate counsel and other
expenses related to such separate counsel to be borne by Arch Coal. ADA-ES shall
not be liable to indemnify Arch Coal, any underwriter or any controlling person
for any settlement of any such action effected without ADA-ES's written consent
(which consent shall not be unreasonably withheld or delayed). ADA-ES shall not,
except with the approval of each party being indemnified under this Section
3.04, consent to entry of any judgment or enter into any settlement which does
not include as an unconditional term thereof the giving by the claimant or
plaintiff to the parties being so indemnified of a release from all liability in
respect to such claim or litigation.

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In order to provide for just and equitable contribution to joint liability under
the Securities Act in any case in which Arch Coal, or any controlling person of
Arch Coal, makes a claim for indemnification pursuant to this Section 3.04 but
it is judicially determined (by the entry of a final judgment or decree by a
court of competent jurisdiction and the expiration of time to appeal or the
denial of the last right of appeal) that such indemnification may not be
enforced in such case notwithstanding the fact that this Section 3.04 provides
for indemnification in such case, then, ADA-ES and Arch Coal will contribute to
the aggregate losses, claims, damages or liabilities to which they may be
subject (after contribution from others) in such proportion as is appropriate to
reflect the relative fault of ADA-ES on the one hand and of Arch Coal on the
other in connection with the statements or omissions which resulted in such
losses, claims, damages or liabilities, as well as any other relevant equitable
considerations. The relative fault of ADA-ES on the one hand and of Arch Coal on
the other shall be determined by reference to, among other things, whether the
untrue or alleged untrue statement of a material fact or omission or alleged
omission to state a material fact relates to information supplied by ADA-ES on
the one hand or by Arch Coal on the other, and each party's relative intent,
knowledge, access to information and opportunity to correct or prevent such
statement or omission; provided, however, that, in any such case, (A) Arch Coal
will not be required to contribute any amount in excess of the public offering
price of all Registrable Shares offered by it pursuant to such registration
statement; and (B) no person or entity guilty of fraudulent misrepresentation
(within the meaning of Section 11(f) of the Securities Act) will be entitled to
contribution from any person or entity who was not guilty of such fraudulent
misrepresentation.

The indemnities provided in this Section 3.04 shall survive the transfer of any
Registrable Shares by Arch Coal.

Section 3.05 Indemnification of Company. In the event that ADA-ES registers any
of the Registrable Shares under the Securities Act, Arch Coal will indemnify and
hold harmless ADA-ES, each of its directors, each of its officers who have
signed or otherwise participated in the preparation of the registration
statement, each underwriter of the Registrable Shares so registered (including
any broker or dealer through whom such of the shares may be sold) and each
person, if any, who controls ADA-ES within the meaning of Section 15 of the
Securities Act from and against any and all losses, claims, damages, expenses or
liabilities, joint or several, to which they or any of them may become subject
under the Securities Act, applicable state securities laws or under any other
statute or at common law or otherwise, and, except as hereinafter provided, will
reimburse ADA-ES and each such director, officer, underwriter or controlling
person for any legal or other expenses reasonably incurred by them or any of
them in connection with investigating or defending any actions whether or not
resulting in any liability, insofar as such losses, claims, damages, expenses,
liabilities or actions arise out of or are based upon any untrue statement or
alleged untrue statement of a material fact contained in the registration
statement, in any preliminary or amended preliminary prospectus or in the final
prospectus (or in the registration statement or prospectus as from time to time
amended or supplemented) or arise out of or are based upon the omission or
alleged omission to state therein a material fact required to be stated therein
or necessary in order to make the statements therein not misleading, but only
insofar as any such statement or omission was made in reliance upon and in
conformity with information furnished in writing to ADA-ES in connection
therewith by Arch Coal expressly for use therein; provided, however, that Arch
Coal's obligations hereunder shall be limited to an amount equal to the proceeds
received by Arch Coal for the Registrable Shares sold in such registration.

Promptly after receipt of notice of the commencement of any action in respect of
which indemnity may be sought against Arch Coal, ADA-ES shall notify Arch Coal
in writing of the commencement thereof (provided, that failure to so notify Arch
Coal shall not relieve Arch Coal from any liability it may have hereunder,
except to the extent prejudiced by such failure), and Arch Coal shall, subject
to the provisions hereinafter stated, be entitled to assume the defense of such
action (including the employment of counsel, who shall be counsel reasonably
satisfactory to ADA-ES) and the payment of expenses insofar as such action shall

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relate to the alleged liability in respect of which indemnity may be sought
against Arch Coal. ADA-ES and each such director, officer, underwriter or
controlling person shall have the right to employ separate counsel in any such
action and to participate in the defense thereof, but the fees and expenses of
such counsel subsequent to any assumption of the defense by Arch Coal shall not
be at the expense of Arch Coal unless employment of such counsel has been
specifically authorized in writing by Arch Coal. Arch Coal shall not be liable
to indemnify any person for any settlement of any such action effected without
Arch Coal's written consent (which consent shall not be unreasonably withheld or
delayed).

In order to provide for just and equitable contribution to joint liability under
the Securities Act in any case in which ADA-ES, its officers, directors or
controlling persons ("ADA-ES Indemnitees") exercising its rights under this
Article III, makes a claim for indemnification pursuant to this Section 3.05,
but it is judicially determined (by the entry of a final judgment or decree by a
court of competent jurisdiction and the expiration of time to appeal or the
denial of the last right of appeal) that such indemnification may not be
enforced in such case notwithstanding that this Section 3.05 provides for
indemnification, in such case, then, ADA-ES Indemnitee and Arch Coal will
contribute to the aggregate losses, claims, damages or liabilities to which they
may be subject (after contribution from others) in such proportion as is
appropriate to reflect the relative fault of ADA-ES Indemnitee on the one hand
and of the Arch Coal on the other in connection with the statements or omissions
which resulted in such losses, claims, damages or liabilities, as well as any
other relevant equitable considerations. The relative fault of ADA-ES Indemnitee
on the one hand and of Arch Coal on the other shall be determined by reference
to, among other things, whether the untrue or alleged untrue statement of a
material fact or omission or alleged omission to state a material fact relates
to information supplied by ADA-ES Indemnitee on the one hand or by Arch Coal on
the other, and each party's relative intent, knowledge, access to information
and opportunity to correct or prevent such statement or omission; provided,
however, that, in any such case, (A) Arch Coal will not be required to
contribute any amount in excess of the public offering price of all such
Registrable Shares offered by it pursuant to such registration statement; and
(B) no person or entity guilty of fraudulent misrepresentation (within the
meaning of Section 11(f) of the Securities Act) will be entitled to contribution
from any person or entity who was not guilty of such fraudulent
misrepresentation.

Section 3.06. Further Obligations of ADA-ES. Whenever under the preceding
Sections of this Article III, ADA-ES is required hereunder to register
Registrable Shares, it agrees that it shall also do the following:

(a) Furnish to Arch Coal such copies of each preliminary and final prospectus
and such other documents as Arch Coal may reasonably request to facilitate the
public offering of the Registrable Shares;

(b) Use its best efforts to register or qualify the Registrable Shares covered
by said registration statement under the applicable securities or "blue sky"
laws of such jurisdictions as Arch Coal may reasonably request; provided,
however, that ADA-ES shall not be obligated to qualify to do business in any
jurisdictions where it is not then so qualified or to take any action which
would subject it to the service of process in suits other than those arising out
of the offer or sale of the securities covered by the registration statement in
any jurisdiction where it is not then so subject;

(c) Permit Arch Coal or its counsel or other representatives to inspect and copy
such corporate documents and records as may reasonably be requested by them,
after reasonable advance notice and without undue interference with the
operation of ADA-ES's business;

(d) Furnish to Arch Coal a copy of all documents filed with and all
correspondence from or to the Commission in connection with any such offering of
securities;

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(e) Use its best efforts to insure the obtaining of all necessary approvals from
the National Association of Securities Dealers, Inc.; and

(f) Otherwise use its best efforts to comply with all applicable rules and
regulations of the Commission, and make available to its security holders, as
soon as reasonably practicable, an earning statement covering the period of at
least twelve months, but not more than eighteen months, beginning with the first
month after the effective date of the registration statement covering a Public
Offering, which earning statement shall satisfy the provisions of Section 11(a)
of the Securities Act and Rule 158 thereunder.

Whenever under the preceding Sections of this Article III Arch Coal is
registering Registrable Shares pursuant to any registration statement, (i) Arch
Coal agrees to timely provide to ADA-ES, at its request, such information and
materials as it may reasonably request in order to effect the registration of
such Registrable Shares and (ii) if the offering is underwritten, ADA-ES and
Arch Coal agree to execute an underwriting agreement containing customary
conditions..

Section 3.07. Expenses. Subject to Section 3.02(b) in the case of each
registration effected under Section 3.01 or 3.02, ADA-ES shall bear its own
reasonable costs and expenses of each such registration on behalf of Arch Coal,
including, but not limited to, ADA-ES's printing, legal and accounting fees and
expenses, Commission and NASD filing fees and "blue sky" fees and expenses;
provided, however, that ADA-ES shall have no obligation to pay or otherwise bear
any portion of the underwriters' commissions or discounts or transfer taxes
attributable to the Registrable Shares being offered and sold by Arch Coal, or
the fees and expenses of counsel for Arch Coal in connection with the
registration of the Registrable Shares.

ADA-ES shall pay all expenses in connection with any registration initiated
pursuant to this Article III which is withdrawn, delayed or abandoned at the
request of ADA-ES, unless such registration is withdrawn, delayed or abandoned
solely because of any action of Arch Coal.

Section 3.08. Non-Transferability. Arch Coal's rights and obligations contained
in this Article III shall not be transferable to any other party under any
circumstances, whether by operation of law or otherwise.

Section 3.09 Lock-Up Agreement. Arch Coal agrees, if so requested by ADA-ES in
connection with any public offering of ADA-ES's securities, not to sell, grant
any option or right to buy or sell, or otherwise transfer or dispose of in any
manner, whether in privately-negotiated or open-market transactions, any Common
Stock or other securities of ADA-ES held by it during the 180-day period
following the effective date of a registration statement filed pursuant to
apublic offering, nor will it permit any of its affiliates or associates to do
any of the foregoing. Arch Coal, its affiliates or associates shall enter into
"lock-up" agreements to such effect. Such "lock-up" agreements shall be in
writing and in form and substance satisfactory to ADA-ES. ADA-ES may impose
stop-transfer instructions with respect to the Shares (or securities) subject to
the foregoing restrictions until the end of said 180-day period.

Section 3.10. Termination of Registration Rights. Notwithstanding any other term
or provision of this Article III, at such time as Arch Coal is free to sell the
Registrable Shares without registration pursuant to Rule 144(k) promulgated
under the Securities Act, all rights of Arch Coal as to such Registrable Shares
under Sections 3.01 and 3.02 of this Article III shall terminate.

                                   ARTICLE IV

                                  MISCELLANEOUS

Section 4.01. Interpretation. For all purposes of this Agreement, the term
ADA-ES Common Stock shall include any securities of any issuer entitled to vote
generally for the election of directors of such issuer which securities the

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holders of ADA-ES Common Stock shall have received or as a matter of right be
entitled to receive as a result of (i) any capital reorganization or
reclassification of the capital stock of ADA-ES, (ii) any consolidation, merger
or share exchange of ADA-ES with or into another corporation or (iii) any sale
or substantially all the assets of ADA-ES.

Section 4.02. Enforcement. (a) Arch Coal acknowledges and agrees that
irreparable damage would occur if any of the provisions of this Agreement were
not performed in accordance with their specific terms or were otherwise
breached. Accordingly, ADA-ES will be entitled to an injunction or injunctions
to prevent breaches of this Agreement and to enforce specifically its provisions
in any court of the United States or any state having jurisdiction, this being
in addition to any other remedy to which ADA-ES may be entitled at law or in
equity.

(b) No failure or delay on the part of ADA-ES in the exercise of any power,
right or privilege hereunder shall operate as a waiver thereof, nor shall any
single or partial exercise of any such power, right or privilege preclude other
or further exercise thereof or of any other right, power or privilege.

Section 4.03. Entire Agreement. This Agreement, together with the applicable
provisions of the Subscription Agreement, constitutes the entire understanding
of the parties with respect to the transactions contemplated hereby and thereby.
This Agreement may be amended only by an agreement in writing executed by ADA-ES
and Arch Coal.

Section 4.04. Severability. If any provision of this Agreement is held by a
court of competent jurisdiction to be unenforceable, the remaining provisions
shall remain in full force and effect. It is declared to be the intention of the
parties that they would have executed the remaining provisions without including
any that may be declared unenforceable.

Section 4.05. Headings. Descriptive headings are for convenience only and will
not control or affect the meaning or construction of any provision of this
Agreement.

Section 4.06. Counterparts. This Agreement may be executed in two or more
counterparts, and each such executed counterpart will be an original instrument.

Section 4.07. Notices. All notices, consents, requests, instructions, approvals
and other communications provided for in this Agreement and all legal process in
regard to this Agreement will be validly given, made or served, if in writing
and delivered personally, by telecopy (except for legal process) or sent by
certified mail postage paid.

if to ADA-ES:              ADA-ES, Inc.
                           8100 SouthPark Way, B
                           Littleton, CO  80120
                           Attn:  President
                           Fax:  (303) 734-0330

if to Arch Coal:           Arch Coal, Inc.
                           One CityPlace Drive
                           Suite 300
                           St. Louis, MO  63141
                           Attn:  David Peugh
                           Fax:  (314) 994-2940

with a copy to:            General Counsel
                           Fax:  (314) 994-2734

or to such other address or telecopy number as any party may, from time to time,
designate in a written notice given in a like manner. Notice by telecopy shall
be deemed delivered on the day telephone confirmation of receipt is given.

                                       8


Section 4.08. Successors and Assigns. This Agreement shall bind the successors
and assigns of the parties, and inure to the benefit of any successor or assign
of any of the parties; provided, however, that no party may assign this
Agreement without the other party's prior written consent; provided further,
however, that the rights contained in Article III of this Agreement may not be
transferred or assigned under any circumstances.

Section 4.09. Legend. Each certificate representing shares of capital stock of
ADA-ES beneficially owned by Arch Coal or its affiliates or associates shall
bear a legend in substantially the following form, until such time as the shares
of capital stock represented thereby are no longer subject to the provisions
hereof:

"The sale, transfer or assignment of the securities represented by this
certificate are subject to the terms and conditions of a certain Standstill and
Registration Rights Agreement dated September 19, 2003, as amended from time to
time, between ADA-ES and Arch Coal, Inc. Copies of such Agreement may be
obtained at no cost by written request made by the holder of record of this
certificate to the Secretary of ADA-ES."

Section 4.10. Governing Law. This Agreement will be governed by and construed
and enforced in accordance with the laws of the State of Colorado, without
giving effect to the conflict of laws principles thereof.

IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be executed
as of the date first referred to above.

ADA-ES, INC.


By:____________________
     Name:  Michael D. Durham
     Title:  President

Arch Coal, Inc.


By:____________________
     Name:
     Title:




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