<PAGE>
 


                                                                     EXHIBIT 4.5

                      FIRST AMENDMENT TO CREDIT AGREEMENT


     THIS FIRST AMENDMENT TO CREDIT AGREEMENT ("First Amendment") dated as of
March 31, 1997 (the "First Amendment Effective Date") is made and entered into
by and among STERLING CHEMICALS, INC., a Delaware corporation (the "Company"),
TEXAS COMMERCE BANK NATIONAL ASSOCIATION, individually, and as Administrative
Agent and CREDIT SUISSE FIRST BOSTON, individually, and as Documentation Agent
and the other financial institutions signatories hereto.


                             INTRODUCTORY STATEMENT
                             ----------------------

     The Company has entered into a credit agreement dated as of June 21, 1996,
among the Company, Texas Commerce Bank National Association, individually, as an
Issuing Bank and as Administrative Agent, Credit Suisse First Boston (formerly
known as Credit Suisse), individually, as an Issuing Bank and as Documentation
Agent, and the financial institutions parties thereto, as amended by the First
Amendment to Credit Agreement (the "First Amendment") dated as of 
January 31, 1997 (as amended by the First Amendment and otherwise, the "Original
Credit Agreement").

     The parties to the Original Credit Agreement and the parties to that
certain Credit Agreement dated as of January 31, 1997, by and among the Company,
Texas Commerce Bank National Association, individually, and as Administrative
Agent, Credit Suisse First Boston, individually, and as Documentation Agent, and
the financial institutions parties thereto (the "Credit Agreement") have entered
into an Intercreditor Agreement dated as of January 31, 1997 (the "Intercreditor
Agreement") pursuant to which the parties thereto agreed to share the proceeds
of prepayments of term loans made by the Company in accordance with the terms
thereof.

     The Company intends to issue at least $150,000,000 of additional senior
subordinated notes and will use the proceeds of such senior subordinated notes
to voluntarily prepay Loans pursuant to Section 2.10(b) of the Credit Agreement
and Section 3.02(b) of the Intercreditor Agreement.

     The Company has requested an increase in the Revolving Credit Commitments
under the Original Credit Agreement in an aggregate amount not to exceed
$25,000,000.

     The Company has requested that certain covenants in the Credit Agreement be
modified and amended.

     The Company, the Administrative Agent, the Documentation Agent, the
Subsidiary Guarantors and the Lenders have agreed, on the terms and conditions
herein set forth, that the Credit Agreement be amended.
<PAGE>
 
                                   AGREEMENT

     Section 1. Definitions.  Capitalized terms used but not otherwise
defined herein shall have the meaning assigned such terms in the Credit
Agreement.

     Section 2. Amendments to the Credit Agreement.  On and after
satisfaction of the conditions set forth in Section 3 below, the Credit
Agreement shall be amended as follows:

     (a)  The following new definitions are hereby added to Section 1.01 of the
          Credit Agreement:

                "Additional Senior Subordinated Notes" shall mean the additional
          senior subordinated notes issued pursuant to the Additional Senior
          Subordinated Notes Indenture, as amended, in an amount of at least
          $150,000,000 issued by the Company.

                "Additional Senior Subordinated Notes Indenture" shall mean that
          certain Sterling Chemicals, Inc. $150,000,000 Senior Subordinated
          Notes Due 2007 Indenture with Fleet National Bank, as Trustee.

                "First Amendment" shall mean the First Amendment to Credit
          Agreement dated as of March 31, 1997, among the Company, the
          Administrative Agent, the Documentation Agent and the Lenders.

                "First Amendment Effective Date" shall mean March 31, 1997.

                "Senior Debt" shall mean, as to any Person, all Funded
          Indebtedness of such Person, less any subordinated Indebtedness with
          terms of subordination substantially similar to the terms of
          subordination of the Senior Subordinated Notes and the Additional
          Senior Subordinated Notes.

                "Senior Debt Leverage Ratio" shall mean, on any day, the ratio
          of (a) Senior Debt of the Company and its Subsidiaries on a
          consolidated basis as of the date of determination to (b) EBITDA for
          the Rolling Period ending on the most recent Quarterly Date as of the
          date of determination.

     (b) The following definitions set forth in Section 1.01 of the Credit
     Agreement are hereby amended in their entirety to read as follows:

                "Agreement" shall mean this Credit Agreement, as amended by the
          First Amendment to Credit Agreement and as further amended, modified
          or supplemented.

                "Cumulative Retained Cash Flow" shall mean an amount equal to
          the difference of (a) Retained Cash Flow for all then previous Fiscal
          Years, on a cumulative basis, minus (b) Designated Retained Cash Flow
          Usage, on a cumulative 

                                       2
<PAGE>
 
          basis, in each case from the date that is the first day to occur after
          the next occurring principal payment required pursuant to Section
          2.05(a) after giving effect to the application of the prepayment by
          the Company of the proceeds of the Additional Senior Subordinated
          Notes pursuant to Section 2.10(b). For the period beginning on August
          22, 1996 and ending on the date that is the first day to occur after
          the next occurring principal payment required pursuant to Section
          2.05(a) after giving effect to the application of the prepayment by
          the Company of the proceeds of the Additional Senior Subordinated
          Notes pursuant to Section 2.10(b), the Cumulative Retained Cash Flow
          shall equal zero.

     (c) Section 2.10(a) of the Credit Agreement is hereby amended by adding a
new clause (v) to read in its entirety as follows:

               (v) Prior to the date that is the first day to occur after the
         next occurring principal payment required pursuant to Section 2.05(a)
         after giving effect to the application of the prepayment by the Company
         of the proceeds of the Additional Senior Subordinated Notes pursuant to
         Section 2.10(b), the Company shall use its Retained Cash Flow to prepay
         or repay first the outstanding Revolving Credit Loans and second to
         prepay the Combined Term Loans, such prepayments to be applied as
         provided in Section 2.10(a)(iv).

     (d) Section 2.10(b) of the Credit Agreement is hereby amended by inserting
the following proviso in the fifth sentence of such Section immediately after
the words "respective installments of principal thereof" as they appear in
clause (i)(A)(y) of such sentence in such Section:

          "; provided, however, that with respect to the proceeds of the
          Additional Senior Subordinated Notes received by the Administrative
          Agent as a prepayment by the Company pursuant to this Section 2.10(b),
          50% of such proceeds shall be applied pro rata to scheduled
          amortization payments of the Original Tranche A Term Loans, the
          Original Tranche B Term Loans and the Loans in the order of maturity
          thereof and the balance of such proceeds shall be applied pro rata to
          the respective remaining installments of principal thereof."

     (e) Section 5.03(a) of the Credit Agreement is hereby amended to read in
its entirety as follows:

          "(a) Interest Coverage Ratio.  Maintain an Interest Coverage Ratio
     of not less than the ratio for each Rolling Period indicated below:

               Each Rolling Period ending                    Ratio

               December 31, 1996                              1.75
               March 31, 1997                                 1.75
               June 30, 1997                                  1.75
               September 30, 1997                             1.50 
 

                                       3
<PAGE>
 
               Each Rolling Period during
               the Fiscal Year ending                        Ratio
 
               September 30, 1998                             1.35
                                                                  
               Each Rolling Period ending                    Ratio
                                                                  
               December 31, 1998                              1.50
               March 31, 1999                                 1.50
               June 30, 1999                                  2.25
               September 30, 1999                             2.25 

               Each Rolling Period                           Ratio
               thereafter                                     2.50"

     (f) Section 5.03(c) of the Credit Agreement is hereby amended to read in
its entirety as follows:
 
          "(c) Fixed Charge Coverage Ratio. Maintain a Fixed Charge Coverage
     Ratio of not less than the ratio for each Rolling Period indicated below:
 
 
               Each Rolling Period during
               the period beginning January 1,               Ratio
               1997 through September 30, 1997                1.05
 
               The Rolling Period ending                     Ratio
               December 31, 1997                              1.00
 
               Each Rolling Period during
               the period beginning January 1,               Ratio
               1998 through September 30, 1998                1.05
 
               Each Rolling Period during
               the period beginning October 1,               Ratio
               1998 through September 30, 1999                1.10
 
               Each Rolling Period during
               the Fiscal Years ending                       Ratio
                          
               September 30, 2000                             1.15
               September 30, 2001                             1.20

               Each Rolling Period                           Ratio
               thereafter                                    1.20"

                                       4
<PAGE>
 
     (g) Section 5.03(d) of the Credit Agreement is hereby amended to read in
its entirety as follows:

          "(d)  Leverage Ratio.  Maintain a Leverage Ratio of not greater than
     the ratio for each Rolling Period indicated below:

               Each Rolling Period ending                    Ratio

               December 31, 1996                              5.25
               March 31, 1997                                 6.35
               June 30, 1997                                  7.00
               September 30, 1997                             7.00
               December 31, 1997                              7.00
               March 31, 1998                                 7.00
               June 30, 1998                                  7.00
               September 30, 1998                             7.00
               December 31, 1998                              6.50
               March 31, 1999                                 6.00
               June 30, 1999                                  4.50
               September 30, 1999                             4.50 
 
               Each Rolling Period during
               the Fiscal Years ending                       Ratio
 
               September 30, 2000                             4.00
               September 30, 2001                             3.50
               September 30, 2002                             3.00 

               Each Rolling Period                           Ratio
               thereafter                                     3.00"

     (h)  Section 5.03 of the Credit Agreement is hereby amended by adding a new
clause (e) to such Section to read in its entirety as follows:
 
          "(e) greater than the ratio for each Rolling Period indicated
     below:        
 
 
               Each Rolling Period ending                    Ratio
 
               March 31, 1997                                 3.50
               June 30, 1997                                  3.50
               September 30, 1997                             3.50
               December 31, 1997                              3.50
               March 31, 1998                                 3.25
               June 30, 1998                                  3.25
               September 30, 1998                             3.25 

                                       5
<PAGE>
 
               Each Rolling Period during
               the Fiscal Years ending                       Ratio
 
               September 30, 1999                             3.00
               September 30, 2000                             3.00
               September 30, 2001                             3.00
               September 30, 2002                             3.00 

               Each Rolling Period                           Ratio
               thereafter                                     3.00"


     (i)  Section 5.04(a) of the Credit Agreement is hereby amended:

          (i) by amending clause (xi) of such Section to read in its entirety as
     follows:

               (i) Indebtedness under the Original Credit Agreement in the
          principal amount not to exceed the sum of the outstanding Original
          Term Loans and the Revolving Credit Commitments (as defined in the
          Original Credit Agreement) less the aggregate amount of permanent
          reductions of the Revolving Credit Commitments and payments or
          prepayments of the Original Term Loans;

          (ii) by deleting the word "and" at the end of clause (x) of such
     Section, by changing the period at the end of clause (xi) to read "; and"
     and by adding the following new clause (xii) to read in its entirety as
     follows:

               (xii) Indebtedness evidenced by the Additional Senior
          Subordinated Notes.

     (j) Sections 5.04(p)(i), (ii) and (iii) of the Credit Agreement are hereby
amended in their entirety as follows:

          (i)  Amend, modify, or waive any covenant contained in the Senior
     Subordinated Notes, the Additional Senior Subordinated Notes, the Senior
     Subordinated Notes Indenture or the Additional Senior Subordinated Notes
     Indenture if the effect of such amendment, modification, or waiver would be
     to make the terms of the Senior Subordinated Notes, the Additional Senior
     Subordinated Notes, the Senior Subordinated Notes Indenture or the
     Additional Senior Subordinated Notes Indenture materially more onerous to
     the Company;

          (ii) Amend, modify, or waive any provision of the Senior Subordinated
     Notes, the Additional Senior Subordinated Notes, the Senior Subordinated
     Notes Indenture or the Additional Senior Subordinated Notes Indenture which
     (A) subjects the Company to any additional material obligation, (B)
     increases the principal of or rate of interest on any Senior Subordinated
     Note or any Additional Senior Subordinated Note, (C) accelerates the date

                                       6
<PAGE>
 
     fixed for any payment of principal or interest on any Senior Subordinated
     Note or any Additional Senior Subordinated Note, (D) would change the
     percentage of holders of such Senior Subordinated Notes or such Additional
     Senior Subordinated Notes required for any such amendment, modification, or
     waiver from the percentage required on the Effective Date, or (E) relates
     to the subordination provisions thereof; or

          (iii)  make any voluntary prepayment of, or optionally redeem, or make
     any payment in defeasance of, any part of the Senior Subordinated Notes or
     the Additional Senior Subordinated Notes.

     (k) Section 6.16 of the Credit Agreement is hereby amended to read in its
entirety as follows:

          Section 6.16  Senior Indebtedness. The Senior Subordinated Notes or
     the Additional Senior Subordinated Notes shall cease, for any reason, to be
     validly subordinated to the Lender Indebtedness, as provided in the Senior
     Subordinated Notes Indenture and Additional Senior Subordinated Notes
     Indenture or the Company, any Affiliate of the Company, the trustee in
     respect of the Senior Subordinated Notes or the Additional Senior
     Subordinated Notes or the holders of at least 25% in aggregate principal
     amount of the Senior Subordinated Notes or the Additional Senior
     Subordinated Notes shall so assert in writing;

       Section 3. Effectiveness of Amendments Contained in Section 2 of this
First Amendment.  If, but only if, the Administrative Agent has received as a
prepayment from the Company pursuant to the provisions of Section 2.10(b) of the
Credit Agreement the proceeds of the Additional Senior Subordinated Notes in an
aggregate principal amount equal to at least $150,000,000, then shall all the
amendments contained in Section 2 hereof become effective.  If the
Administrative Agent does not receive such prepayment from the Company then the
amendments contained in Section 2 of this First Amendment shall be of no force
and effect whatsoever.

     Section 4.   Limitations.  The amendments set forth herein are limited
precisely as written and shall not be deemed to (a) be a consent to, or waiver
or modification of, any other term or condition of the Credit Agreement or any
of the other Financing Documents, or (b) except as expressly set forth herein,
prejudice any right or rights which the Lenders may now have or may have in the
future under or in connection with the Credit Agreement, the Financing Documents
or any of the other documents referred to therein.  Except as expressly modified
hereby or by express written amendments thereof, the terms and provisions of the
Credit Agreement, the Notes, and any other Financing Documents or any other
documents or instruments executed in connection with any of the foregoing are
and shall remain in full force and effect.  In the event of a conflict between
this First Amendment and any of the foregoing documents, the terms of this First
Amendment shall be controlling.

       Section 5. Conditions Precedent and Effectiveness.  This First
Amendment shall not be effective until: (a) this First Amendment has been
executed and delivered by the Required Lenders; (b) a certified copy of the
resolutions of the Board of Directors of the Company dated as of the First
Amendment Effective Date has been delivered to the Administrative Agent, such
resolutions 

                                       7
<PAGE>
 
approving, the prepayment described in Section 3 of this First Amendment, the
Additional Senior Subordinated Notes, this First Amendment, the Second Amendment
to Original Credit Agreement, and all other documents, if any, to which the
Company is a party and evidencing corporate authorization with respect to such
documents; (c) the delivery of an opinion of counsel from Andrews & Kurth
L.L.P., addressed to each of the Agents and the Lenders and covering such other
matters as any Administrative Agent or the Lenders may reasonably request; and
(d) the execution and delivery of this First Amendment by the Subsidiary
Guarantors and the Company.

      Section 6.  Representations and Warranties.  Except as affected by the
transactions contemplated in the Credit Agreement and this First Amendment, each
of the representations and warranties made by the Company and the Subsidiary
Guarantors in or pursuant to the Financing Documents, including the Credit
Agreement, shall be true and correct in all material respects as of the First
Amendment Effective Date, as if made on and as of such date.

      Section 7.  No Default.  No Default or Event of Default shall have
occurred and be continuing as of the First Amendment Effective Date.

     Section 8.   Approval of Revolving Credit Commitment Increase under
Original Credit Agreement.  Each of the Company, the Administrative Agent, the
Documentation Agent and the Lenders signatories hereto hereby approve and
consent to the increase in the Original Lenders' Revolving Credit Commitment by
an aggregate amount not to exceed $25,000,000.

      Section 9.  Adoption, Ratification and Confirmation of Credit Agreement.
Each of the Company, the Administrative Agent, the Documentation Agent, and the
Lenders signatories hereto does hereby adopt, ratify and confirm the Credit
Agreement, as amended hereby, and acknowledges and agrees that the Credit
Agreement, as amended hereby, is and remains in full force and effect.

      Section 10. Ratification and Affirmation of Subsidiary Guaranty.  Each
of the Subsidiary Guarantors hereby expressly (i) acknowledges the terms of this
First Amendment, (ii) acknowledges, renews and extends its continued liability
under the Guaranty Agreement dated as of January 31, 1997, in favor of the
Agents, the Issuing Banks (as defined in the Original Credit Agreement), the
Lenders, TCB as administrative agent under the Original Credit Agreement, the
Documentation Agent (as defined in the Original Credit Agreement) and the
Original Lenders, as amended, supplemented or otherwise modified, and agrees
that such Guaranty Agreement remains in full force and effect; and (iii)  agrees
with the Administrative Agent, the Documentation Agent, each Lender, the Issuing
Banks (as defined in the Original Credit Agreement) TCB as administrative agent
under the Original Credit Agreement, the Documentation Agent (as defined in the
Original Credit Agreement and each Original Lender to promptly pay when due all
amounts owing or to be owing by it under such Guaranty Agreement pursuant to the
terms and conditions thereof.  Additionally, Sterling Fibers hereby expressly
(i) acknowledges the terms of this First Amendment, (ii) acknowledges, renews
and extends its continued liability under the Limited Guaranty Agreement (Santa
Rosa) dated as of January 31, 1997, in favor of the Agents, the Issuing Banks
(as defined in the Original Credit Agreement), the Lenders, TCB as
administrative agent under the Original Credit Agreement, the Documentation
Agent (as defined in the Original Credit Agreement) and the Original Lenders, as
amended, supplemented or otherwise modified, and agrees that such Limited
Guaranty 

                                       8
<PAGE>
 
Agreement remains in full force and effect; and (iii) agrees with the
Administrative Agent, the Documentation Agent, each Issuing Bank, each Lender,
TCB as administrative agent under the Original Credit Agreement, the
Documentation Agent (as defined in the Original Credit Agreement and each
Original Lender to promptly pay when due all amounts owing or to be owing by it
under such Limited Guaranty Agreement pursuant to the terms and conditions
thereof.

     Section 11.  Payment of Expenses.  The Company agrees, whether or not the
transactions hereby contemplated shall be consummated, to reimburse and save the
Administrative Agent and the Documentation Agent  harmless from and against
liability for the payment of all reasonable out-of-pocket costs and expenses
arising in connection with the preparation, execution, delivery, amendment,
modification, waiver and enforcement of, or the preservation of any rights under
this First Amendment, including, without limitation, the reasonable fees and
expenses of any local or other counsel for the Administrative Agent, and all
stamp taxes (including interest and penalties, if any), recording taxes and
fees, filing taxes and fees, and other charges which may be payable in respect
of, or in respect of any modification of, the Credit Agreement and the other
Financing Documents.  The provisions of this Section shall survive the
termination of the Credit Agreement and the repayment of the Loans.

     Section 12.    Governing Law.  THIS FIRST AMENDMENT AND THE RIGHTS AND
OBLIGATIONS OF THE PARTIES HEREUNDER AND UNDER THE CREDIT AGREEMENT SHALL BE
CONSTRUED IN ACCORDANCE WITH AND BE GOVERNED BY THE LAWS OF THE STATE OF NEW
YORK (INCLUDING SECTION 5-1401 OF THE NEW YORK GENERAL OBLIGATIONS LAW, OR ANY
SIMILAR SUCCESSOR PROVISIONS THERETO, BUT EXCLUDING ALL OTHER CONFLICT-OF-LAWS
RULES) AND TO THE EXTENT CONTROLLING, LAWS OF THE UNITED STATES OF AMERICA.

     Section 13.  Descriptive Headings, etc.  The descriptive headings of the
several Sections of this First Amendment are inserted for convenience only and
shall not be deemed to affect the meaning or construction of any of the
provisions hereof.

     Section 14.  Entire Agreement.  This First Amendment and the documents
referred to herein represent the entire understanding of the parties hereto
regarding the subject matter hereof and supersede all prior and contemporaneous
oral and written agreements of the parties hereto with respect to the subject
matter hereof, including, without limitation, any commitment letters regarding
the transactions contemplated by this First Amendment.

     Section 15.  Counterparts.  This First Amendment may be executed in any
number of counterparts (including by telecopy) and by different parties on
separate counterparts and all of such counterparts shall together constitute one
and the same instrument.

     Section 16.  Amendment to Intercreditor Agreement.  The Administrative
Agent is hereby authorized and directed to execute and deliver on behalf of the
Lenders an amendment to the Intercreditor Agreement of even date herewith in the
form of Exhibit A to this First Amendment.

                                       9
<PAGE>
 
     IN WITNESS WHEREOF, the parties hereto have caused this First Amendment to
be duly executed and delivered by their respective duly authorized officers as
of the date first above written.


COMPANY:                      STERLING CHEMICALS, INC.

 


                              By:__________________________________________
                                 Jim P. Wise
                                 Vice President and Chief Financial Officer



ADMINISTRATIVE AGENT          TEXAS COMMERCE BANK
DOCUMENTATION AGENT           NATIONAL ASSOCIATION,
AND THE LENDERS:              Individually and as Administrative Agent



                              By:__________________________________________
                                 Name:
                                 Title:


                              CREDIT SUISSE FIRST BOSTON,
                              Individually and as Documentation Agent



                              By:__________________________________________
                                 Name:
                                 Title:


                              By:__________________________________________
                                 Name:
                                 Title:



                              [Signature Page-1]
<PAGE>
 
                              ABN AMRO BANK N.V.
                              Houston Agency

                              By:   ABN AMRO NORTH AMERICA, INC.
                                    as Agent


                                    By: ___________________________________
                                         Name:
                                         Title:


                                    By: ___________________________________
                                         Name:
                                         Title:


                              THE BANK OF NOVA SCOTIA


                              By: _________________________________________
                                  Name:
                                  Title:


                              BANK OF SCOTLAND


                              By: _________________________________________
                                  Name:
                                  Title:
 

                              [Signature Page-2]
<PAGE>
 
                              BHF-BANK AKTIENGESELLSCHAFT


                              By: _________________________________________
                                  Name:
                                  Title:


                              By: _________________________________________
                                  Name:
                                  Title:


                              THE FIRST NATIONAL BANK OF CHICAGO


                              By: _________________________________________
                                  Name:
                                  Title:


                              THE CIT GROUP/BUSINESS CREDIT, INC.


                              By: _________________________________________
                                  Name:
                                  Title:


                              HIBERNIA NATIONAL BANK


                              By: _________________________________________
                                  Name:
                                  Title:



                              [Signature Page-3]
<PAGE>
 
                              NATIONAL BANK OF CANADA


                              By: _________________________________________
                                  Name:
                                  Title:


                              By: _________________________________________
                                  Name:
                                  Title:


                              OCTAGON CREDIT INVESTORS LOAN PORTFOLIO (a unit of
                              The Chase Manhattan Bank),


                              By: _________________________________________
                                  Name:
                                  Title:


                              MERRILL LYNCH SENIOR FLOATING RATE FUND, INC.


                              By: _________________________________________
                                  Name:
                                  Title:


                              ML CBO IV (CAYMAN) LTD.

                                     By: PROTECTIVE ASSET MANAGEMENT L.L.C. 
                                         AS COLLATERAL MANAGER


                                     By: __________________________________
                                         Name:
                                         Title:



                              [Signature Page-4]
<PAGE>
 
                              VAN KAMPEN AMERICAN CAPITAL PRIME 
                              RATE INCOME TRUST


                              By: _________________________________________
                                  Name:
                                  Title:


                              PARIBAS CAPITAL FUNDING LLC


                              By: _________________________________________
                                  Name:
                                  Title:


                              CAPTIVA II FINANCE LTD.


                              By: _________________________________________
                                  Name:
                                  Title:


                              MORGAN STANLEY SENIOR FUNDING, INC.


                              By: _________________________________________
                                  Name:
                                  Title:


                              MERRILL LYNCH PRIME RATE PORTFOLIO

                                    By:  Merrill Lynch Asset Management, L.P.
                                         as Investment Advisor


                                    By: ___________________________________
                                        Name:
                                        Title:




                              [Signature Page-5]
<PAGE>
 
                              SENIOR DEBT PORTFOLIO

                                    By:  Boston Management and Research,
                                         as Investment Advisor


                                    By: ___________________________________
                                        Name:
                                        Title:


                              SKANDINAVIKA ENSKILDA BANKEN
                              CORPORATION



                              By: _________________________________________
                                  Name:
                                  Title:


                              THE CHASE MANHATTAN BANK



                              By: _________________________________________
                                  Name:
                                  Title:



                              [Signature Page-6]
<PAGE>
 
SUBSIDIARY GUARANTORS:        STERLING CHEMICALS INTERNATIONAL, INC.
                              STERLING CHEMICALS ENERGY, INC.


                              By: _________________________________________
                                  Jim P. Wise
                                  Vice President



                              STERLING CANADA, INC.
                              STERLING PULP CHEMICALS US, INC.
                              STERLING PULP CHEMICALS, INC.
                              STERLING FIBERS, INC.


                              By: _________________________________________
                                  Jim P. Wise
                                  Vice President - Finance



                              [Signature Page-7]
<PAGE>
 
                                   EXHIBIT A
                                       TO
                      FIRST AMENDMENT TO CREDIT AGREEMENT

                                   [FORM OF]
                  [FIRST AMENDMENT TO INTERCREDITOR AGREEMENT]


     THIS FIRST AMENDMENT TO INTERCREDITOR AGREEMENT ("First Amendment") dated
as of March 31, 1997 (the "First Amendment Effective Date") is made and entered
into by and among (i) the banks and other financial institutions (the "Original
Lenders") which are or may from time to time become parties to the Original
Credit Agreement (as hereinafter defined) , TEXAS COMMERCE BANK NATIONAL
ASSOCIATION, as their Administrative Agent (the "Original Administrative
Agent"); (ii) the banks and other financial institutions (the "AFB Lenders")
which are or may  from time to time become parties to the AFB Acquisition Credit
Agreement (as hereinafter defined), by TEXAS COMMERCE BANK NATIONAL ASSOCIATION,
as their administrative agent (the "AFB Administrative Agent") to the AFB
Acquisition Credit Agreement (as hereinafter defined) and STERLING CHEMICALS,
INC., a Delaware corporation (the "Company").


                             INTRODUCTORY STATEMENT
                             ----------------------

     The Company has entered into a credit agreement dated as of June 21, 1996,
among the Company, Texas Commerce Bank National Association, individually, as an
Issuing Bank and as Administrative Agent, Credit Suisse First Boston (formerly
known as Credit Suisse), individually, as an Issuing Bank and as Documentation
Agent, and the financial institutions parties thereto, as amended by the First
Amendment to Credit Agreement dated as of January 31, 1997 (as amended from time
to time, the "Original Credit Agreement").

     The parties to the Original Credit Agreement and the parties to that
certain Credit Agreement dated as of January 31, 1997, by and among the Company,
Texas Commerce Bank National Association, individually, and as Administrative
Agent, Credit Suisse First Boston, individually, and as Documentation Agent, and
the financial institutions parties thereto (as amended, the "AFB Acquisition
Credit Agreement") have entered into an Intercreditor Agreement dated as of
January 31, 1997 (the "Intercreditor Agreement") pursuant to which the parties
thereto agreed to share the proceeds of prepayments of term loans made by the
Company in accordance with the terms thereof.

     The Company intends to issue at least $150,000,000 of additional senior
subordinated notes and will use the proceeds of such senior subordinated notes
to voluntarily prepay the Combined Term Loans (as defined in the Intercreditor
Agreement) pursuant to Section 2.10(c) of the Original Credit Agreement, Section
2.10(b) of the AFB Acquisition Credit Agreement and Section 3.02(b) of the
Intercreditor Agreement.
<PAGE>
 
     The Company has requested that certain covenants in the AFB Acquisition
Credit Agreement be modified and amended and the Original Administrative Agent
and the Administrative Agents, acting on behalf of the Combined Lenders (as
defined in the Intercreditor Agreement) have determined that corresponding
changes and modifications need to be made in the Intercreditor Agreement.

     The Company, the AFB Administrative Agent, the Original Administrative
Agent and the Combined Lenders have agreed, on the terms and conditions herein
set forth, that the Intercreditor Agreement be amended.

                                   AGREEMENT

     Section 1.     Definitions.  Capitalized terms used but not otherwise
defined herein shall have the meaning assigned such terms in the Intercreditor
Agreement.

     Section 2.     Amendments to the Intercreditor Agreement.  On and after
satisfaction of the conditions set forth in Section 3 below, the Intercreditor
Agreement shall be amended as follows:

     (a)  The following new definitions are hereby added to Section 1.01 of the
          Intercreditor Agreement:

               "First Amendment to AFB Acquisition Credit Agreement" shall mean
          the First Amendment to Credit Agreement dated as of March 31, 1997,
          among the AFB Administrative Agent, the Documentation Agent under the
          AFB Acquisition Credit Agreement,  the AFB Lenders signatories thereto
          and the Company.

               "First Amendment to Intercreditor Agreement" shall mean the First
          Amendment to Intercreditor Agreement dated as of March 31, 1997, among
          the AFB Administrative Agent, the Original Administrative Agent, the
          Combined Lenders, and the Company.

               "First Amendment Effective Date" shall mean March 31, 1997.

               "Second Amendment to Original Credit Agreement" shall mean the
          Second Amendment to Credit Agreement dated as of March 31, 1997, among
          the Original Adminstrative Agent, the Original Lenders signatories
          thereto, the Documentation Agent under the Original Credit Agreement
          and the Company.

     (b) The following definitions set forth in Section 1.01 of the
     Intercreditor Agreement are hereby amended in their entirety to read as
     follows:

               "AFB Acquisition Credit Agreement" shall mean the credit
          agreement dated as of even date herewith by and among the Company, the
          AFB Lenders, the AFB Administrative Agent, and Credit Suisse First
          Boston, as documentation agent, as 

                                       2
<PAGE>
 
          amended by the First Amendment to AFB Acquisition Credit Agreement and
          as amended, supplemented or modified from time to time.

               "Intercreditor Agreement" shall mean this Intercreditor
          Agreement, as amended by the First Amendment to Intercreditor
          Agreement and as further amended, modified or supplemented.

               "Original Credit Agreement" shall mean the credit agreement dated
          as of June 21, 1996, by and among the Company, the Lenders, the
          Original Administrative Agent and Credit Suisse First Boston (formerly
          known as Credit Suisse), as documentation agent, as amended by the
          First Amendment to Original Credit Agreement, the Second Amendment to
          Original Credit Agreement, and as further amended, supplemented or
          modified from time to time.

     (c) Section 3.02(b) of the Intercreditor Agreement is hereby amended by
inserting the following proviso in the fifth sentence of such Section
immediately after the words "respective installments of principal thereof" as
they appear in clause (i)(A)(y) of such sentence in such Section:

     "; provided, however, that with respect to the proceeds of the Additional
     Senior Subordinated Notes received by the Administrative Agents as a
     prepayment by the Company pursuant to Section 2.10(b) of the AFB
     Acquisition Credit Agreement, Section 2.10(c) of the Original Credit
     Agreement and this Section 3.02.(b), 50% of such proceeds shall be applied
     pro rata to scheduled amortization payments of the Original Tranche A Term
     Loans, the Original Tranche B Term Loans and the AFB Loans in the order of
     maturity thereof and the balance of such proceeds shall be applied pro rata
     to the respective remaining installments of principal thereof."

      Section 3.   Effectiveness of Amendments Contained in Section 2 of this
First Amendment.  If, but only if, the Administrative Agents has received as a
prepayment from the Company pursuant to the provisions of  Section 2.10(b) of
the AFB Acquisition Credit Agreement, Section 2.10(c) of the Original Credit
Agreement and Section 3.02(b) of the Intercreditor Agreement the proceeds of the
Additional Senior Subordinated Notes in an aggregate principal amount equal to
at least $150,000,000, then shall all the amendments contained in Section 2
hereof become effective.  If the Administrative Agents do not receive such
prepayment from the Company then the amendments contained in Section 2 of this
First Amendment shall be of no force and effect whatsoever.

     Section 4.     Limitations.  The amendments set forth herein are limited
precisely as written and shall not be deemed to (a) be a consent to, or waiver
or modification of, any other term or condition of the Intercreditor Agreement
or any of the other Financing Documents, or (b) except as expressly set forth
herein, prejudice any right or rights which the Combined Lenders may now have or
may have in the future under or in connection with the Intercreditor Agreement,
the Financing Documents or any of the other documents referred to therein.
Except as expressly modified hereby or by express written amendments thereof,
the terms and provisions of the Intercreditor Agreement or any other documents
or instruments executed in connection therewith are and shall remain in full

                                       3
<PAGE>
 
force and effect.  In the event of a conflict between this First Amendment and
any of the foregoing documents, the terms of this First Amendment shall be
controlling.

       Section 5.   Conditions Precedent and Effectiveness.  This First
Amendment shall not be effective until: (a) the First Amendment to AFB
Acquisition Credit Agreement has been executed and delivered by the Required
Lenders thereof; (b) the Second Amendment to Original Credit Agreement has been
executed and delivered by the Required Lenders thereof; (c) this First Amendment
has been executed and delivered by Texas Commerce Bank National Association,
acting in its capacity as the Original Administrative Agent (and in such
capacity as Administrative Agent for the Combined Lenders) and on behalf of the
Original Lenders and as the AFB Administrative Agent and on behalf of the AFB
Lenders; and (d) this First Amendment has been joined and delivered by the
Company.

      Section 6.    No Default.  No Default or Event of Default under either of
the Original Credit Agreement or the AFB Acquisition Credit Agreement shall have
occurred and be continuing as of the First Amendment Effective Date.

      Section 7.    Adoption, Ratification and Confirmation of Intercreditor
Agreement.  Each of the Company, the Administrative Agents, the Documentation
Agents, and the Combined Lenders does hereby adopt, ratify and confirm the
Intercreditor Agreement, as amended hereby, and acknowledges and agrees that the
Intercreditor Agreement, as amended hereby, is and remains in full force and
effect.

     Section 8.     Payment of Expenses.  The Company agrees, whether or not the
transactions hereby contemplated shall be consummated, to reimburse and save the
Administrative Agents and the Documentation Agents  harmless from and against
liability for the payment of all reasonable out-of-pocket costs and expenses
arising in connection with the preparation, execution, delivery, amendment,
modification, waiver and enforcement of, or the preservation of any rights under
this First Amendment, including, without limitation, the reasonable fees and
expenses of any local or other counsel for the Administrative Agents, and all
stamp taxes (including interest and penalties, if any), recording taxes and
fees, filing taxes and fees, and other charges which may be payable in respect
of, or in respect of any modification of, the Intercreditor Agreement and the
other Financing Documents.  The provisions of this Section shall survive the
termination of the Intercreditor Agreement and the repayment of the Combined
Loans.

     Section 9.     Governing Law.  THIS FIRST AMENDMENT AND THE RIGHTS AND
OBLIGATIONS OF THE PARTIES HEREUNDER AND UNDER THE INTERCREDITOR AGREEMENT SHALL
BE CONSTRUED IN ACCORDANCE WITH AND BE GOVERNED BY THE LAWS OF THE STATE OF NEW
YORK (INCLUDING SECTION 5-1401 OF THE NEW YORK GENERAL OBLIGATIONS LAW, OR ANY
SIMILAR SUCCESSOR PROVISIONS THERETO, BUT EXCLUDING ALL OTHER CONFLICT-OF-LAWS
RULES) AND TO THE EXTENT CONTROLLING, LAWS OF THE UNITED STATES OF AMERICA.

     Section 10.    Descriptive Headings, etc.  The descriptive headings of the
several Sections of this First Amendment are inserted for convenience only and
shall not be deemed to affect the meaning or construction of any of the
provisions hereof.

                                       4
<PAGE>
 
     Section 11.    Entire Agreement.  This First Amendment and the documents
referred to herein represent the entire understanding of the parties hereto
regarding the subject matter hereof and supersede all prior and contemporaneous
oral and written agreements of the parties hereto with respect to the subject
matter hereof, including, without limitation, any commitment letters regarding
the transactions contemplated by this First Amendment.

     Section 12.    Counterparts.  This First Amendment may be executed in any
number of counterparts (including by telecopy) and by different parties on
separate counterparts and all of such counterparts shall together constitute one
and the same instrument.

                                       5
<PAGE>
 
     IN WITNESS WHEREOF, the parties hereto have caused this First Amendment to
be duly executed and delivered by their respective duly authorized officers as
of the date first above written.

                                    TEXAS COMMERCE BANK NATIONAL
                                    ASSOCIATION, as Administrative
                                    Agent under the Original Credit
                                    Agreement (and in such capacity, as
                                    Administrative Agent For Combined Lenders) 
                                    and on behalf of the Original Lenders and 
                                    as Administrative Agent under the AFB
                                    Acquisition Credit Agreement and on behalf 
                                    of the AFB Lenders



                                    By:_________________________________
                                          Name:
                                          Title:

JOINDER:

STERLING CHEMICALS, INC.
(as Company under the Original
Credit Agreement and under the
AFB Acquisition Credit Agreement)



By:______________________________
      Name:  Jim P. Wise
      Title:  Vice President and Chief Financial Officer


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