Document:

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                                                                    Exhibit 10.8

THIS PROMISSORY NOTE HAS NOT BEEN REGISTERED UNDER THE U.S. SECURITIES ACT OF
1933 ("THE ACT"), NOR UNDER APPLICABLE STATE SECURITIES LAWS. THIS NOTE MAY NOT
BE OFFERED FOR SALE, SOLD OR OTHERWISE TRANSFERRED EXCEPT PURSUANT TO AN
EFFECTIVE REGISTRATION STATEMENT UNDER THE ACT OR PURSUANT TO AN EXEMPTION FROM
REGISTRATION UNDER THE ACT AND STATE LAWS, THE AVAILABILITY OF WHICH IS TO BE
ESTABLISHED TO THE SATISFACTION OF THE COMPANY.

ISSUE DATE:  February 14, 2001

AMOUNT:      $_________                                          Note No. A-____

                                NGE LEASING, INC.
              SERIES A 10% SUBORDINATED NOTE DUE DECEMBER 31, 2006

1.    PROMISE TO PAY

1.1   Promise to Pay - FOR VALUE RECEIVED, NGE LEASING, INC., a Texas
corporation with its principal executive office at 2911 South CR 1260, Midland,
Texas 79706 (the "Company"), promises to pay to the order of _____________,
located at __________ (the "Lender"), on December 31, 2006 or such date
specified below (the "Maturity Date"), the principal sum of $_______ (the
"Principal Sum"). Interest at the rate of 10% per annum on the Principal Sum
will be compounded and payable annually, with the first interest payment due
December 31, 2001.

1.2   Maturity Date - The Maturity Date of this Series A 10% Subordinated Note
(the "Note") is December 31, 2006. The Company shall pay the Principal Sum
outstanding to the Lender in lawful money of the United States of America on the
Maturity Date at the address of the Lender set out above or such other address
as the Lender designates by written notice to the Company prior to the payment
being made. This Note may be pre-paid in whole or in part at any time without
penalty.

1.3   Events of Default - The whole of the Principal Sum or the balance
remaining unpaid, together with any accrued and unpaid interest may, at the
option of the Lender, become immediately due and payable upon the occurrence of
any of the following events (each event being called an "event of default"):

      (a)   the Company defaults in payment of the Principal Sum on the Maturity
            Date and the default continues for 30 days after written notice of
            the default to the Company by the Lender;

      (b)   the Company has not paid an annual interest payment for any year
            when due and the default continues for 60 days after written notice
            of the default to the Company by the Lender;

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      (c)   the Company defaults in the performance or observance of any other
            covenant or condition of the Note and the default continues for 60
            days after written notice of the default to the Company by the
            Lender;

      (d)   an order is made for the winding-up of the Company; a petition is
            filed by or against the Company; an assignment for the benefit of
            creditors is made by the Company; a receiver or agent is appointed
            in respect of the Company under any bankruptcy or insolvency
            legislation, or by or on behalf of a secured creditor of the
            Company; or an application is made under the United States
            Bankruptcy Code or any successor or similar legislation;

      (e)   the Company ceases to carry on its business or disposes of
            substantially all of its assets; or

      (f)   the Company takes any corporate proceedings for its dissolution or
            liquidation.

1.4   Unsecured Loan; Subordinated to Prior Debt - This Note is not secured by
collateral. The indebtedness represented by this Note is subordinate to secured
indebtedness of the Company.

1.5   Guaranty of Debt - The payment of principal and interest under this Note
is guaranteed by Natural Gas Services Group, Inc. in the event NGE Leasing, Inc.
defaults on the payment of principal or interest.

2.    REGISTERS OF THE COMPANY

2.1   The Company shall, at all times while this Note is outstanding, cause to
be kept by and at the principal office of the Company, registers in which will
be entered the Lender's name and address and particulars of this Note held by
it. No transfer, exchange or conversion of this Note will be valid unless made
by the Lender or its administrators or other legal representatives or its
attorney duly appointed by an instrument in writing unless in form and execution
satisfactory to the Company acting reasonably upon compliance with such
requirements as are set out in this Note, and such other requirements as the
Company acting reasonably may prescribe, and unless the transfer has been duly
entered on one of the appropriate registers or noted on this Note by the Company
or other registrar.

2.2   The registers referred to above will at all reasonable times be open for
inspection by the Lender.

2.3   Subject to restrictions under the Securities Act of 1933 and similar laws,
the Lender may at any time and from time to time have this Note transferred at
any of the places at which a register is kept pursuant to the provisions of this
section, in accordance with such reasonable regulations as the Company may
prescribe.

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3.    GENERAL

3.1   Ownership of Note - The Company will not be charged with notice of nor be
bound to see to the execution of any trust, whether expressed, implied or
constructive, in respect of this Note and the Company may transfer this Note on
the direction of the Lender whether named as trustee or otherwise, as though
that person were the beneficial owner.

3.2   Notice and Other Instruments - Any notice, demand or other communication
required or permitted to be given to a party must be in writing and must be:

      (a)   personally delivered to that party or a director or officer of that
            party; or

      (b)   except during a period of strike, lock-out or other postal
            disruption, sent by registered mail, postage prepaid to the address
            of that party set forth on page one; or

      (c)   sent by telegraph, telecopier or telex or similar communication
            tested prior to sending and confirmed by prepaid registered or
            certified mail to the address of that party set forth on page one;

and will be deemed to have been received by that party on the earliest of the
date of delivery under paragraph (a), the actual date of receipt where mailed
under paragraph (b) and the day following the date of communication (otherwise
than by mail) under paragraph (c). Any party may give written notice to the
other party of a change of address to some other address, in which event any
communication must thereafter be given to that party, at the last such changed
address of which the party communicating has received written notice.

3.3   Headings - Headings to the sections, paragraphs, subparagraphs and clauses
of this Note have been inserted for convenience of reference only, and are not
to affect its construction.

3.4   Governing Law - This Note and the rights, remedies, powers, covenants,
duties and obligations of the parties will be construed in accordance with and
governed by the laws of the State of Colorado and the federal laws of the United
States.

3.5   Arbitration - Any controversy, claim, dispute and matters of difference
with respect to this Agreement and the transactions contemplated by it must be
resolved through submission to arbitration in Denver, Colorado according to the
rules and practices of the American Arbitration Association from time to time in
force.

3.6   Severability - If any provision of this Note is or becomes invalid,
illegal or unenforceable in any respect, that fact will not affect the validity,
legality or enforceability of the remaining provisions of this Note or any
valid, legal or enforceable parts of the impugned provision.

3.7   Binding on Successors - This Note will inure to the benefit of and be
binding upon each of the parties and their respective heirs, executors,
administrators, successors and permitted assigns.

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3.8   Amendment and Waiver - This Note may not be amended, waived, discharged or
terminated except by a document executed by the party against whom enforcement
of the amendment, waiver, discharge or termination is sought.

3.9   Entire Agreement - This Note, together with the Subscription Agreement and
Investment Letter between Natural Gas Services Group, Inc. and the Lender, sets
out the entire agreement and understanding of the Company and the Lender with
respect to the loan and supersedes all prior oral and written agreements,
undertakings and understandings.

                                                NGE LEASING, INC.

                                                By:_____________________________

                                                Title:_______________________

                                                GUARANTOR:

                                                NATURAL GAS SERVICES GROUP, INC.

                                                By:_____________________________

                                                Title:_______________________

                                       4<PAGE>
                                                                    Exhibit 10.9

THE WARRANTS AND UNDERLYING SHARES OF COMMON STOCK REPRESENTED BY THIS
CERTIFICATE HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933 (THE
"ACT"), AND ARE "RESTRICTED SECURITIES" AS THAT TERM IS DEFINED IN RULE 144
UNDER THE ACT. THE SECURITIES MAY NOT BE OFFERED FOR SALE, SOLD OR OTHERWISE
TRANSFERRED EXCEPT PURSUANT TO AN EFFECTIVE REGISTRATION STATEMENT UNDER THE
ACT, OR PURSUANT TO AN EXEMPTION FROM REGISTRATION UNDER THE ACT, THE
AVAILABILITY OF WHICH IS TO BE ESTABLISHED TO THE SATISFACTION OF THE COMPANY.

                                                    Warrant Certificate No. ____
                                                  Issue Date:  February 14, 2001

                       WARRANT TO PURCHASE ________ SHARES
            VOID AFTER 5:00 P.M., MOUNTAIN TIME, ON DECEMBER 31, 2006

                        NATURAL GAS SERVICES GROUP, INC.
                        WARRANT AGREEMENT AND CERTIFICATE

      This certifies that, for value received, ______________, located at
__________, the registered holder hereof (the "Warrant Holder"), is entitled to
purchase from Natural Gas Services Group, Inc., a Colorado corporation (the
"Company") with its principal office located in Midland, Texas, at any time
commencing on the Issue Date set forth above, and before 5:00 P.M., Mountain
Time, on December 31, 2006 (the "Termination Date") at the purchase price of
$3.25 per share (the "Exercise Price"), the number of shares of the Company's
Common Stock (the "Shares") set forth above. The number of Shares purchasable
upon exercise of this Warrant and the Exercise Price per Share shall be subject
to adjustment from time to time as set forth in Section 5 below.

Section 1.  Sale and Delivery of Warrant. The Warrants represented hereby were
issued in connection with the private offering commenced in October 2000 of the
Company's Series A 10% Subordinated Notes due December 31, 2006 (the "Private
Offering").

Section 2.  Transfer or Exchange of Warrant.

2.1   The Company shall be entitled to treat the registered owner of any Warrant
(the "Warrant Holder") as the owner in fact thereof for all purposes and shall
not be bound to recognize any equitable or other claim to or interest in such
Warrant on the part of any other person, and shall not be liable for any
registration of transfer of Warrants which are registered or to be registered in
the name of a fiduciary or the nominee of a fiduciary unless made with the
actual knowledge that a fiduciary or nominee is committing a breach of trust in
requesting such registration of transfer, or with such knowledge of such facts
that its participation therein amounts to gross negligence or bad faith.

2.2   This Warrant may not be sold, transferred, assigned or hypothecated except
pursuant to all applicable federal and state securities laws.

2.3   A Warrant shall be transferable only on the books of the Company upon
delivery of this Warrant Certificate duly endorsed by the Warrant Holder or by
his duly authorized attorney or representative, or accompanied by proper
evidence of succession, assignment or authority to transfer.

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Upon any registration of transfer, the Company shall deliver a new Warrant
Certificate to the persons entitled thereto.

Section 3. Term of Warrants; Exercise of Warrants.

3.1   Subject to the terms of this Agreement and Certificate, the Warrant Holder
has the right, which may be exercised commencing immediately and ending at 5:00
p.m. Mountain Time on the Termination Date, to purchase from the Company the
number of Shares which the Warrant Holder may at that time be entitled to
purchase on exercise of this Warrant.

3.2   A Warrant shall be exercised by surrender to the Company, at its principal
office, of this Warrant Agreement and Certificate, together with the form of
election to purchase attached hereto as Exhibit A, duly completed and signed,
and payment to the Company of the Exercise Price (as defined in accordance with
the provisions of Section 4 hereof) for the number of Shares in respect of which
such Warrant is then exercised. Except as provided in Section 3.3, payment of
the aggregate Exercise Price shall be made in cash or certified funds.

3.3   In addition to and without limiting the rights of the Holder under the
terms of this Warrant, the Holder shall have the right (the "Conversion Right")
to convert this Warrant or any portion thereof into Shares as provided in this
Section 3.3 at any time prior to the Termination Date.

            a. Upon exercise of the Conversion Right with respect to a
particular number of Shares (the "Converted Shares"), the Company shall deliver
to the Holder, without payment by the Holder of any Exercise Price or any cash
or other consideration, that number of Shares equal to the quotient obtained by
dividing the Net Value (as hereinafter defined) of the Converted Shares by the
fair market value (as defined in paragraph 3.3(c) below) of a single Share,
determined in each case as of the close of business on the Conversion Date (as
hereinafter defined). The "Net Value" of the Converted Shares shall be
determined by subtracting the aggregate Exercise Price of the Converted Shares
from the aggregate fair market value of the Converted Shares. No fractional
securities shall be issuable upon exercise of the Conversion Right, and if the
number of securities to be issued in accordance with the foregoing formula is
other than a whole number, the Company shall pay to the Holder an amount in cash
equal to the fair market value of the resulting fractional Share.

            b. The Conversion Right may be exercised by the Holder by the
surrender of this Warrant at the principal office of the Company or at the
office of the Company's stock transfer agent, if any, together with a written
statement specifying that the Holder thereby intends to exercise the Conversion
Right and indicating the number of Shares subject to the Warrant which are being
surrendered (referred to in subparagraph 3.3(a) above as the Converted Shares),
in the form attached to and by this reference incorporated in this Warrant as
Exhibit B, in exercise of the Conversion Right. Such conversion shall be
effective upon receipt by the Company of this Warrant together with the
aforesaid written statement, or on such later date as is specified therein (the
"Conversion Date"), but not later than the expiration date of the Warrant.
Certificates for the Converted Shares issuable upon exercise of the Conversion
Right, together with a check in payment of any fractional Warrant Share and, in
the case of a partial exercise a new warrant evidencing the Warrant Shares
remaining

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subject to the Warrant, shall be issued as of the Conversion Date and shall be
delivered to the Holder within twenty-one days following the Conversion Date.

            c. For purposes of this Section 2.3, the "fair market value" of a
Share as of a particular date shall be its "market price," which, for purposes
of this Section 2.3 shall mean, if the Shares are traded on a securities
exchange or on Nasdaq, the closing price of the Shares on such exchange or the
last sale price on Nasdaq, or, if the Shares are otherwise traded in the
over-the-counter market, the average of the closing bid and ask price, in each
case averaged over a period of five consecutive trading days prior to the date
as of which "market price" is being determined. If at any time the Shares are
not traded on an exchange or Nasdaq, or otherwise traded in the over-the-counter
market, the "market price" shall be deemed to be the fair value thereof
determined in good faith by the Board of Directors of the Company as of a date
which is within 15 days of the date as of which the determination is to be made.

3.4   Subject to Section 5 hereof, upon surrender of a Warrant Agreement and
Certificate and payment of the Exercise Price as aforesaid, the Company shall
issue and cause to be delivered with all reasonable dispatch to or upon the
written order of the Warrant Holder exercising such Warrant and in such name or
names as such Warrant Holder may designate, certificates for the number of
Shares so purchased upon the exercise of such Warrant. Such certificate or
certificates shall be deemed to have been issued and any person so designated to
be named therein shall be deemed to have become a holder of record of such
Shares as of the date of receipt by the Company of such Warrant Agreement and
Certificate and payment of the Exercise Price. The rights of purchase
represented by the Warrants shall be exercisable, at the election of the Warrant
Holders thereof, either in full or from time to time in part and, in the event
that a Warrant Agreement and Certificate is exercised to purchase less than all
of the Shares purchasable on such exercise at any time prior to the Termination
Date, a new Warrant Agreement and Certificate evidencing the remaining Warrant
or Warrants will be issued.

3.5   The Warrant Holder will pay all documentary stamp taxes, if any,
attributable to the initial issuance of the Shares upon the exercise of
Warrants.

Section 4. Adjustment of Exercise Price and Shares.

4.1   If the Company shall at any time subdivide its outstanding Common Stock by
recapitalization, reclassification or split-up thereof, the number of Shares of
Common Stock subject to this Warrant immediately prior to such subdivision shall
be proportionately increased, and if the Company shall at any time combine the
outstanding Common Stock by recapitalization, reclassification or combination
thereof, the number of Shares of Common Stock subject to this Warrant
immediately prior to such combination shall be proportionately decreased. Any
corresponding adjustment to the Exercise Price shall become effective at the
close of business on the record date for such subdivision or combination.

4.2   In the event of a dividend (other than in shares of Common Stock of the
Company), the proposed dissolution or liquidation of the Company, or any
corporate separation or division, including, but not limited to, a split-up,
split-off or spin-off, or a merger or consolidation of the Company with another
corporation, or the sale of all or substantially all of the assets of the

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Company, the Board may provide that each Warrantholder will have the right to
exercise this Warrant (at its then current Exercise Price) solely for the kind
and amount of shares of stock and other securities, property, cash or any
combination thereof receivable upon such dissolution, liquidation, corporate
separation or division, or merger or consolidation by a holder of the number of
shares of Common Stock for which this Warrant might have been exercised
immediately prior to such dissolution, liquidation, corporate separation or
division, or merger or consolidation; or, in the alternative, the Board may
provide that this Warrant will terminate as of a date fixed by the Board;
provided, however, that not less than 30 days' written notice of the date so
fixed must be given to the Warrantholder, who will have the right, during the
period of 30 days preceding such termination, to exercise this Warrant as to all
or any part of the shares of Common Stock covered by this Warrant.

4.3   The preceding paragraph will not apply to a merger or consolidation in
which the Company is the surviving corporation and shares of Common Stock are
not converted into or exchanged for stock, securities of any other corporation,
cash or any other thing of value. Notwithstanding the preceding sentence, in
case of any consolidation or merger of another corporation into the Company in
which the Company is the surviving corporation and in which there is a
reclassification or change (including a change to the right to receive cash or
other property) of the shares of Common Stock (excluding a change in par value,
or from no par value to par value, or any change as a result of a subdivision or
combination, but including any change in such shares into two or more classes or
series of shares), the Board may provide that the holder of this Warrant will
have the right to exercise this Warrant solely for the kind and amount of shares
of stock and other securities (including those of any new direct or indirect
parent of the Company), property, cash or any combination thereof receivable
upon such reclassification, change, consolidation or merger by the holder of the
number of shares of Common Stock for which this Warrant might have been
exercised.

4.4   In the event of a change in the Common Stock of the Company as presently
constituted into the same number of shares with a different par value, the
shares resulting from any such change will be deemed to be the Common Stock of
the Company within the meaning of this agreement.

4.5   To the extent that the foregoing adjustments relate to stock or securities
of the Company, such adjustments will be made in good faith by the Board.

4.6   Except as expressly provided in this Warrant, the Warrantholder will have
no rights by reason of any subdivision or consolidation of shares of stock of
any class, or the payment of any stock dividend or any other increase or
decrease in the number of shares of stock of any class, or by reason of any
dissolution, liquidation, merger, or consolidation or spin-off of assets or
stock of another corporation; and any issue by the Company of shares of stock of
any class, or securities convertible into shares of stock of any class, will not
affect, and no adjustment will be made with respect to, the number or price of
shares of Common Stock subject to this Warrant. The grant of this Warrant will
not affect in any way the right or power of the Company to make adjustments,
reclassifications, reorganizations or changes of its capital or business
structures, or to merge or consolidate, or to dissolve, liquidate, or sell or
transfer all or any part of its business or assets.

Section 5. Mutilated or Missing Warrant Certificates. In case any Warrant
Certificate shall be mutilated, lost, stolen or destroyed, the Company shall, at
the request of the holder of such

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<PAGE>

Certificate, issue and deliver, in exchange and substitution for and upon
cancellation of the mutilated Certificate, or in lieu of and substitution for
the Certificate, lost, stolen or destroyed, a new Warrant Certificate of like
tenor and representing an equivalent right or interest; but only upon receipt of
evidence satisfactory to the Company of such loss, theft or destruction of such
Warrant Certificate and indemnity, if requested, also satisfactory to the
Company. An applicant for such a substitute Warrant Certificate shall also
comply with such other reasonable regulations and pay such other reasonable
charges as the Company may prescribe.

Section 6. Reservation of Shares of Common Stock. There has been reserved, and
the Company shall at all times keep reserved so long as any of the Warrants
remain outstanding, out of its authorized Common Stock a number of shares of
Common Stock sufficient to provide for the exercise of the rights of purchase
represented by the outstanding Warrants and the underlying securities.

Section 7. No Fractional Shares. The Company shall not be required to issue
fractional shares or scrip representing fractional shares upon the exercise of
the Warrants. As to any final fraction of a Share which the Warrant Holder would
otherwise be entitled to purchase upon such exercise, the Company shall pay a
cash adjustment in respect of such final fraction in an amount equal to the same
fraction of the market price of a share of Common Stock on the business day
preceding the day of exercise.

Section 8. Transfer and Exercise to Comply With the Securities Act of 1933.

8.1   Neither the Warrants nor the Warrant Shares may be sold, transferred or
otherwise disposed of except to a person, who, in the opinion of counsel for the
Company, is a person to whom such securities may legally be transferred pursuant
to the provisions of this Agreement without registration, and without the
delivery of a current prospectus, under the Act with respect thereto.

8.2   Unless the Company has on file with the Securities and Exchange Commission
a current registration statement to permit the Company to issue registered
shares upon exercise of the Warrants, the Warrants may not be exercised except
in a transaction exempt from registration under the Act.

8.3   The Company shall cause the following legend to be set forth on each
Warrant Certificate and certificates representing the Warrant Shares, unless
counsel for the Company is of the opinion as to any such Certificates that such
legend is unnecessary:

            The securities represented by this certificate may not be offered
            for sale, sold or otherwise transferred except pursuant to an
            effective registration statement made under the Securities Act of
            1933 (the "Act"), or pursuant to an exemption from registration
            under the Act the availability of which is to be established to the
            satisfaction of the Company.

Section 9. Notices. Any notice pursuant to this Agreement by the Company or by
the Warrant Holders shall be in writing and shall be deemed to have been duly
given if delivered or mailed

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certified mail, return receipt requested, (a) if to the Company, to Natural Gas
Services Group, Inc., 2911 So. County Road 1260, Midland, TX 79706, attn:
President; and (b) if to the Warrant Holder, to the address set forth above.
Each party hereto may from time to time change the address to which notices to
it are to be delivered or mailed hereunder by notice in accordance herewith to
the other party.

Section 11. Successors. All the covenants and provisions of this Agreement by or
for the benefit of the Company or the Warrant Holders shall bind and inure to
the benefit of their respective successors and assigns hereunder.

Section 12. Applicable Law. This Warrant Agreement and Certificate and any
replacement Certificate issued hereunder shall be deemed to be a contract made
under the laws of the State of Colorado and for all purposes shall be construed
in accordance with the laws of said State.

Section 13. Benefits of This Agreement. Nothing in this Agreement shall be
construed to give to any person or corporation other than the Company and the
Warrant Holders any legal or equitable right, remedy or claim under this
Agreement and this Agreement shall be for the sole and exclusive benefit of the
Company and the Warrant Holders.

DATED: _________________, 2002                  NATURAL GAS SERVICES GROUP, INC.

                                                By:_____________________________
                                                   Wayne L. Vinson, President

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<PAGE>

                                                                       EXHIBIT A

                                  PURCHASE FORM

                                                     DATED _______________, ____

      THE UNDERSIGNED HEREBY IRREVOCABLY ELECTS TO EXERCISE THE WARRANT
REPRESENTED BY THIS WARRANT CERTIFICATE TO THE EXTENT OF PURCHASING __________
SHARES OF NATURAL GAS SERVICES GROUP, INC. AND HEREBY MAKES PAYMENT OF $3.25 PER
SHARE IN PAYMENT OF THE EXERCISE PRICE THEREOF.

                     INSTRUCTIONS FOR REGISTRATION OF STOCK

NAME:

________________________________________________________________________________
                    (PLEASE TYPE OR PRINT IN BLOCK LETTERS)

ADDRESS:

________________________________________________________________________________

SIGNATURE______________________________________________________

DATED: ___________________,______

--------------------------------------------------------------------------------

                                 ASSIGNMENT FORM

      FOR VALUE RECEIVED, __________________________, HEREBY SELLS, ASSIGNS AND
TRANSFERS UNTO

NAME:

________________________________________________________________________________
                     (PLEASE TYPE OR PRINT IN BLOCK LETTERS)

ADDRESS:

________________________________________________________________________________
THE RIGHT TO PURCHASE SHARES OF NATURAL GAS SERVICES GROUP, INC. REPRESENTED BY
THIS WARRANT CERTIFICATE TO THE EXTENT OF _________ SHARES AS TO WHICH SUCH
RIGHT IS EXERCISABLE AND DOES HEREBY IRREVOCABLY CONSTITUTE AND APPOINT NATURAL
GAS SERVICES GROUP, INC. TO TRANSFER THE SAME ON THE BOOKS OF THE COMPANY WITH
FULL POWER OF SUBSTITUTION IN THE PREMISES.

__________________________________________________
SIGNATURE

DATED:___________________,______

                                        NOTICE: THE SIGNATURE OF THIS ASSIGNMENT
                                        MUST CORRESPOND WITH THE NAME AS IT
                                        APPEARS UPON THE FACE OF THIS WARRANT
                                        CERTIFICATE IN EVERY PARTICULAR, WITHOUT
                                        ALTERATION OR ENLARGEMENT OR ANY CHANGE
                                        WHATEVER.

<PAGE>

                                                                       EXHIBIT B

                        WARRANT CONVERSION EXERCISE FORM

TO: NATURAL GAS SERVICES GROUP, INC.

      Pursuant to Section 2.3 of the Warrant Certificate from Natural Gas
Services Group, Inc. (the "Company") to the undersigned Holder, the Holder
hereby irrevocably elects to convert Warrants with respect to Shares of the
Company into ___________ Shares of the Company.

A conversion calculation is attached hereto as Exhibit B-1.

      The undersigned requests that certificates for such Shares be issued as
follows:

     Name:        __________________________________________

     Address:     __________________________________________

                  __________________________________________

     Deliver to: __________________________________________

and that a new Warrant Certificate for the balance remaining of the Warrants, if
any, subject to the Warrant be registered in the name of, and delivered to, the
undersigned at the address stated above.

     Dated: _________________

                                                    ____________________________
                                                    [NAME OF WARRANT HOLDER]

<PAGE>

                                                                     EXHIBIT B-1

                        CALCULATION OF WARRANT CONVERSION

Shares to be Delivered by Company    =    Net Value
                                          _________
                                            fmv

                         fmv         =    $____________

                         Net Value   =    aggregate fmv of Converted Shares -
                                          aggregate Exercise Price of Converted
                                          Shares

                                     =    $____________ - $____________

                                     =    $____________

  Shares to be Delivered by Company  =    _____________

                  Fractional Shares  =    ____________

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