Document:

Exhibit 10.4

                                                         $8,000,000.00 Term Note
                                                           dated January 3, 2005

                             MODIFICATION AGREEMENT

This Modification Agreement ("Modification Agreement") is effective as of May 1,
2005. The parties to the Modification  Agreement are Natural Gas Services Group,
Inc. ("Borrower") and Western National Bank ("Lender").

                                    RECITALS

On January 3, 2005,  Borrower executed and delivered to Lender that certain Term
Promissory Note in the original principal sum of $8,000,000.00, bearing interest
at the rate stated therein, with a stated final maturity date of January 1, 2012
(the "Note"), pursuant to that certain Third Amended and Restated Loan Agreement
dated  January 3, 2005,  and  subsequently  amended and restated in that certain
Fourth  Amended  and  Restated  Loan  Agreement  dated March 14, 2005 (the "Loan
Agreement"). All liens, security interests and assignments securing the Note are
collectively called the "Liens". Terms defined in the Note or the Loan Agreement
and not otherwise  defined  herein shall have the same meanings here as in those
documents.

At  Borrower's  request,  Borrower  and  Lender  have  agreed to enter into this
Modification  Agreement  to modify the  interest  provisions  of the Note and to
ratify the Liens. Said modified interest provisions shall be effective as of May
1, 2005.

                                    AGREEMENT

1.  Modification  of Interest  Provisions  of the Note. In lieu of the following
provisions which were contained in the first paragraph of the Note:

                  "...at a rate per annum  which  shall from day to day be equal
         to the lesser of (a) a rate per annum (the "Established Rate") equal to
         the greater of (i) one percent  (1%) over the Prime Rate in effect from
         day to day, or (ii) six percent (6.0%), or (b) the Highest Lawful Rate,
         in each  case  calculated  on the  basis of actual  days  elapsed,  but
         computed as if each calendar year consisted of 360 days."

such provisions of the Note are changed to read in their entirety as follows:

                  "...at a rate per annum  which  shall from day to day be equal
         to the lesser of (a) a rate per annum (the "Established Rate") equal to
         the  greater  of (i)  one-half  percent  (0.5%)  over the Prime Rate in
         effect from day to day, or (ii) six percent (6.0%),  or (b) the Highest
         Lawful  Rate,  in each case  calculated  on the  basis of  actual  days
         elapsed, but computed as if each calendar year consisted of 360 days."

                                                                               1
<PAGE>

3.  Ratification  of Liens.  Borrower  and Lender  further  agree that all Liens
securing  the Note  shall  continue  and  carry  forward  until the Note and all
indebtedness  evidenced  thereby is paid in full.  Borrower  further agrees that
such liens are hereby ratified and affirmed as valid and subsisting  against the
collateral described therein,  and that this Modification  Agreement shall in no
manner  vitiate,  affect or impair the Note or the Liens  (except  as  expressly
modified in this  Modification  Agreement)  and that such Liens shall not in any
manner be waived, released, altered or modified.

4. Miscellaneous.

         (a)      As modified  hereby,  the provisions of the Note and the Liens
                  shall  continue  in  full  force  and  effect,   and  Borrower
                  acknowledges  and affirms its liability to Lender  thereunder.
                  In the event of an  inconsistency  between  this  Modification
                  Agreement  and the  terms  of the Note or of the  Liens,  this
                  Modification Agreement shall govern.

         (b)      Borrower hereby agrees to pay all costs and expenses  incurred
                  by Lender in connection with the execution and  administration
                  of this Modification Agreement.

         (c)      Any default by Borrower in the performance of its obligations
                  herein contained shall constitute a default under the Note and
                  the Liens and shall allow Lender to exercise any or all of its
                  remedies set forth in such Note and Liens or at law or in
                  equity.

         (d)      Lender  does  not,  by  its  execution  of  this  Modification
                  Agreement, waive any rights it may have against any person not
                  a party hereto.

         (e)      All terms, provisions,  covenants,  agreements, and conditions
                  of the Note and the Liens are  unchanged,  except as  provided
                  herein.  Borrower agrees that this Modification  Agreement and
                  all of the covenants and agreements  contained herein shall be
                  binding upon Borrower and shall inure to the benefit of Lender
                  and  each  of  their  respective   heirs,   executors,   legal
                  representatives, successors, and permitted assigns.

                  THIS  MODIFICATION  AGREEMENT  REPRESENTS THE FINAL  AGREEMENT
                  BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE OF
                  PRIOR, CONTEMPORANEOUS,  OR SUBSEQUENT ORAL AGREE MENTS OF THE
                  PARTIES.  THERE ARE NO UNWRITTEN ORAL  AGREEMENTS  BETWEEN THE
                  PARTIES.

                                                                               2
<PAGE>

                                     Borrower:
                                     ---------

                                     Natural Gas Services Group, Inc.

                                     By: /s/ Stephen C. Taylor
                                        ----------------------------------------
                                        Stephen C. Taylor, President

                                     Lender:
                                     -------

                                     Western National Bank

                                     By /s/ Scott A. Lovett
                                       -----------------------------------------
                                       Scott A. Lovett, Executive Vice President

STATE OF TEXAS        ss.
                      ss.
COUNTY OF MIDLAND     ss.

         This instrument was acknowledged  before me on May 10, 2005, by Stephen
C.  Taylor,   President  of  Natural  Gas  Services  Group,   Inc.,  a  Colorado
corporation, on behalf of said corporation.

                                        /s/ Deborah K. Tullos
                                       -----------------------------------------
                                       Notary Public, State of Texas

STATE OF TEXAS        ss.
                      ss.
COUNTY OF MIDLAND     ss.

         This instrument was acknowledged before me on May 11, 2005, by Scott A.
Lovett,  Executive Vice President of Western  National Bank, a national  banking
association, on behalf of said association.

                                        /s/ Deborah K. Tullos
                                       -----------------------------------------
                                       Notary Public, State of Texas

                                                                               3Exhibit 10.5

                                                      Revolving Line Credit Note
                                                           dated January 3, 2005

                             MODIFICATION AGREEMENT

This Modification Agreement ("Modification Agreement") is effective as of May 1,
2005. The parties to the Modification  Agreement are Natural Gas Services Group,
Inc. ("Borrower") and Western National Bank ("Lender").

                                    RECITALS

On January 3, 2005,  Borrower  executed  and  delivered  to Lender that  certain
Revolving  Line of  Credit  Promissory  Note in the  original  principal  sum of
$2,000,000.00,  bearing interest at the rate stated therein, with a stated final
maturity  date of January 1, 2006 (the  "Note"),  pursuant to that certain Third
Amended and Restated  Loan  Agreement  dated  January 3, 2005,  as  subsequently
amended and restated in that certain  Fourth Amended and Restated Loan Agreement
dated March 14, 2005 (the "Loan Agreement").  All liens,  security interests and
assignments securing the Note are collectively called the "Liens". Terms defined
in the Note or the Loan  Agreement and not otherwise  defined  herein shall have
the same meanings here as in those documents.

At  Borrower's  request,  Borrower  and  Lender  have  agreed to enter into this
Modification  Agreement  to modify the  interest  provisions  of the Note and to
ratify the Liens. Said modified interest provisions shall be effective as of May
1, 2005.

                                    AGREEMENT

1.  Modification  of Interest  Provisions  of the Note. In lieu of the following
provisions which were contained in the first paragraph of the Note:

                  "...at a rate per annum  which  shall from day to day be equal
         to the lesser of (a) a rate per annum (the "Established Rate") equal to
         the greater of (i) one percent  (1%) over the Prime Rate in effect from
         day to day, or (ii) six percent (6.0%), or (b) the Highest Lawful Rate,
         in each  case  calculated  on the  basis of actual  days  elapsed,  but
         computed as if each calendar year consisted of 360 days."

such provisions of the Note are changed to read in their entirety as follows:

                  "...at a rate per annum  which  shall from day to day be equal
         to the lesser of (a) a rate per annum (the "Established Rate") equal to
         the  greater  of (i)  one-half  percent  (0.5%)  over the Prime Rate in
         effect from day to day, or (ii) six percent (6.0%),  or (b) the Highest
         Lawful  Rate,  in each case  calculated  on the  basis of  actual  days
         elapsed, but computed as if each calendar year consisted of 360 days."

                                                                               1
<PAGE>

3.  Ratification  of Liens.  Borrower  and Lender  further  agree that all Liens
securing  the Note  shall  continue  and  carry  forward  until the Note and all
indebtedness  evidenced  thereby is paid in full.  Borrower  further agrees that
such liens are hereby ratified and affirmed as valid and subsisting  against the
collateral described therein,  and that this Modification  Agreement shall in no
manner  vitiate,  affect or impair the Note or the Liens  (except  as  expressly
modified in this  Modification  Agreement)  and that such Liens shall not in any
manner be waived, released, altered or modified.

4. Miscellaneous.

         (a)      As modified  hereby,  the provisions of the Note and the Liens
                  shall  continue  in  full  force  and  effect,   and  Borrower
                  acknowledges  and affirms its liability to Lender  thereunder.
                  In the event of an  inconsistency  between  this  Modification
                  Agreement  and the  terms  of the Note or of the  Liens,  this
                  Modification Agreement shall govern.

         (b)      Borrower hereby agrees to pay all costs and expenses  incurred
                  by Lender in connection with the execution and  administration
                  of this Modification Agreement.

         (c)      Any default by Borrower in the  performance of its obligations
                  herein contained shall constitute a default under the Note and
                  the Liens and shall allow Lender to exercise any or all of its
                  remedies  set  forth in such  Note  and  Liens or at law or in
                  equity.

         (d)      Lender  does  not,  by  its  execution  of  this  Modification
                  Agreement, waive any rights it may have against any person not
                  a party hereto.

         (e)      All terms, provisions,  covenants,  agreements, and conditions
                  of the Note and the Liens are  unchanged,  except as  provided
                  herein.  Borrower agrees that this Modification  Agreement and
                  all of the covenants and agreements  contained herein shall be
                  binding upon Borrower and shall inure to the benefit of Lender
                  and  each  of  their  respective   heirs,   executors,   legal
                  representatives, successors, and permitted assigns.

                  THIS  MODIFICATION  AGREEMENT  REPRESENTS THE FINAL  AGREEMENT
                  BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE OF
                  PRIOR, CONTEMPORANEOUS,  OR SUBSEQUENT ORAL AGREE MENTS OF THE
                  PARTIES.  THERE ARE NO UNWRITTEN ORAL  AGREEMENTS  BETWEEN THE
                  PARTIES.

                                                                               2
<PAGE>

                                     Borrower:
                                     ---------

                                     Natural Gas Services Group, Inc.

                                     By: /s/ Stephen C. Taylor
                                        ----------------------------------------
                                        Stephen C. Taylor, President

                                     Lender:
                                     -------

                                     Western National Bank

                                     By /s/ Scott A. Lovett
                                       -----------------------------------------
                                       Scott A. Lovett, Executive Vice President

STATE OF TEXAS        ss.
                      ss.
COUNTY OF MIDLAND     ss.

         This instrument was acknowledged  before me on May 10, 2005, by Stephen
C.  Taylor,   President  of  Natural  Gas  Services  Group,   Inc.,  a  Colorado
corporation, on behalf of said corporation.

                                        /s/ Deborah K. Tullos
                                       -----------------------------------------
                                       Notary Public, State of Texas

STATE OF TEXAS        ss.
                      ss.
COUNTY OF MIDLAND     ss.

         This instrument was acknowledged before me on May 11, 2005, by Scott A.
Lovett,  Executive Vice President of Western  National Bank, a national  banking
association, on behalf of said association.

                                        /s/ Deborah K. Tullos
                                       -----------------------------------------
                                       Notary Public, State of Texas

                                                                               3

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