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                                                                     Exhibit 4.3

                          SECOND SUPPLEMENTAL INDENTURE

              THIS SECOND SUPPLEMENTAL INDENTURE (this "Second Supplemental
Indenture") dated as of March 16, 1998 is between STERLING CHEMICALS HOLDINGS,
INC., a Delaware corporation (the "Company"), and STATE STREET BANK AND TRUST
COMPANY, as Trustee (in such capacity, the "Trustee").

                             PRELIMINARY STATEMENTS

       1.     The Company and Fleet National Bank, as Trustee (the "Initial
              Trustee"), entered into that certain Indenture dated as of August
              15, 1996 (as amended, the "Indenture") providing, among other
              things, for the issuance of the Company's 13 1/2% Senior Secured
              Discount Notes Due 2008 in the aggregate principal amount of up to
              $191,751,000 (the "Discount Notes").

       2.     The Trustee has replaced the Initial Trustee as the "Trustee"
              under the Indenture.

       3.     Pursuant to Section 10.01 of the Indenture, the Company and the
              Trustee may amend or supplement the Indenture without the consent
              of any Holder of a Discount Note to, among other things, cure any
              ambiguity, omission, defect or inconsistency.

       4.     The Company has determined that Section 5.05(b)(v) of the
              Indenture contains a defect in that it fails to permit the Company
              or any of its Restricted Subsidiaries to repurchase shares of the
              common stock of the Company that have been distributed to
              participants and beneficiaries of a terminated employee stock
              ownership plan of the Company and Sterling Chemicals, Inc. which
              was in existence as of the date of the Indenture, as required by
              the terms of such plan and applicable law, while permitting
              repurchases under similar circumstances from participants and
              beneficiaries of Sterling Chemicals, Inc.'s current employee stock
              ownership plan.

       5.     Pursuant to Section 10.01 of the Indenture, upon the request of
              the Company accompanied by a resolution of the Board of Directors
              of the Company authorizing the execution of any such amended or
              supplemental Indenture, and upon receipt of certain documents by
              the Trustee, the Trustee is required to join with the Company in
              the execution of any amended or supplemental Indenture authorized
              or permitted by the terms of the Indenture.

       6.     The Board of Directors of the Company has authorized the execution
              of this Second Supplemental Indenture and, simultaneously with and
              as a condition to the execution of this Second Supplemental
              Indenture by the Trustee, the Company is delivering to the Trustee
              resolutions of the Board of Directors of the Company authorizing
              the execution of this Second Supplemental Indenture and an
              Officers' Certificate and Opinion of Counsel, each in the form
              required by the Indenture and satisfactory to the Trustee.


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       7.     All acts and things necessary to make the Indenture, as
              supplemented and modified hereby, a valid, binding and legal
              agreement according to its terms have been done and performed.

              NOW, THEREFORE, in consideration of the premises and other good
and valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the Company covenants and agrees with the Trustee, for the equal
and proportionate benefit of the respective Holders from time to time of the
Discount Notes, as follows:

                                    ARTICLE I

                             Amendment of Indenture

              Clause (v) of Section 5.05(b) of the Indenture is hereby amended
to read in its entirety as follows:

              (v) repurchases of common stock of Holdings distributed to
       participants and beneficiaries of any employee stock ownership plan of
       Holdings or any of its Restricted Subsidiaries which was in effect as of
       the Issue Date as required by and in accordance with the terms thereof
       and Section 409(h)(1)(B) of the Code and the regulations thereunder;
       provided, however, that such amount shall be excluded in the calculation
       of the amount of Restricted Payments;

                                   ARTICLE II

                                   The Trustee

              The Trustee hereby accepts the trusts declared and provided in
this Second Supplemental Indenture, upon the terms and conditions contained in
the Indenture as amended hereby. All of the provisions of the Indenture with
respect to the rights, privileges, immunities, powers and duties of the Trustee
shall be applicable in respect of this Second Supplemental Indenture as fully
and with the same effect as if set forth herein in full. The Trustee shall not
be responsible for and makes no representation as to the validity or adequacy of
this Second Supplemental Indenture, the authorization or permissibility of this
Second Supplemental Indenture pursuant to the terms of the Indenture, the due
execution hereof by the Company or for or in respect of the recitals or
statements contained herein (other than the statements contained in the first
sentence of this Article II), all of such recitals and statements being made
solely by the Company. The Trustee shall not be responsible in any manner to
determine the correctness of provisions contained in this Second Supplemental
Indenture.


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                                   ARTICLE III

                     Effect of Execution and Delivery Hereof

              From and after the execution and delivery of this Second
Supplemental Indenture, (i) the Indenture shall be deemed to be amended and
modified as provided herein, (ii) this Second Supplemental Indenture shall form
a part of the Indenture, (iii) except as modified and amended by this Second
Supplemental Indenture, the Indenture shall continue in full force and effect,
and (iv) every Holder of Discount Notes heretofore and hereafter authenticated
and delivered under the Indenture shall be bound by this Second Supplemental
Indenture.

                                   ARTICLE IV

                            Miscellaneous Provisions

              Section 4.01 Defined Terms. Capitalized terms used but not defined
in this Second Supplemental Indenture shall have the respective meanings
ascribed to such terms in the Indenture.

              Section 4.02 Headings. The headings of the Articles and Sections
of this Second Supplemental Indenture have been inserted for convenience only,
and shall not, for any reason, be deemed to be part of this Second Supplemental
Indenture and shall in no way modify or restrict any of the terms or provisions
hereof.

              Section 4.03 Counterparts. This Second Supplemental Indenture may
be executed in any number of counterparts and by the different parties hereto on
separate counterparts, each of which when so executed and delivered shall be an
original, but all of which shall together constitute one and the same
instrument.

              Section 4.04 Successors and Assigns. All agreements of the Company
contained in this Second Supplemental Indenture shall bind the Company and its
successors. All agreements of the Trustee in this Second Supplemental Indenture
shall bind the Trustee and its successors. This Second Supplemental Indenture
shall be binding upon and inure to the benefit of and be enforceable by the
Holders of any Discount Notes then Outstanding.

              Section 4.05 Governing Law. THIS SECOND SUPPLEMENTAL INDENTURE
SHALL BE GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF
NEW YORK WITHOUT GIVING EFFECT TO APPLICABLE PRINCIPLES OF CONFLICT OF LAW TO
THE EXTENT THAT THE APPLICATION OF THE LAW OF ANOTHER JURISDICTION WOULD BE
REQUIRED THEREBY.


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              IN WITNESS WHEREOF, the parties hereto have caused this Second
Supplemental Indenture to be duly executed, and their respective corporate seals
to be hereunto affixed and attested, all as of the day and year first above
written.

                                      STERLING CHEMICALS HOLDINGS, INC.

             [SEAL]

                                      By:   /s/ ROBERT W. ROTEN
                                           -------------------------------------
                                           Robert W. Roten
                                           President and Chief Executive Officer


ATTEST:



By: /s/ GARY M. SPITZ
   ----------------------------------------------
      Gary M. Spitz
      Vice President and Chief Financial Officer




                                      STATE STREET BANK AND TRUST COMPANY,
                                        AS TRUSTEE

             [SEAL]

                                      By: /s/ MICHAEL M. HOPKINS
                                         -------------------------------------
                                      Printed Name: Michael M. Hopkins
                                                    --------------------------
                                      Title:   Vice President
                                              --------------------------------



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