<PAGE>
 
                                                                     EXHIBIT 3.1


                                                              [State of Delaware
                                                              Secretary of State
                                                        Division of Corporations
                                                       Filed 09:50 AM 08/21/1996
                                                              960243742-2090990]



                             CERTIFICATE OF MERGER
                                    MERGING
                             STX ACQUISITION CORP.
                                     INTO
                           STERLING CHEMICALS, INC.

    (Pursuant to Section 251 of the General Corporation Law of the State of
                                   Delaware)

     STERLING CHEMICALS INC., a Delaware corporation ("Sterling"), hereby
certifies that:

  1. STX Acquisition Corp., a Delaware corporation ("Acquisition"), and Sterling
are the constituent corporations to the merger of Acquisition with and into
Sterling (the "Merger").

  2. An Amended and Restated Agreement and Plan of Merger (the "Merger
Agreement") in connection with the Merger has been approved, adopted, certified,
executed and acknowledged by each of Acquisition and Sterling in accordance with
the provisions of Section 251 of the General Corporation Law of the State of
Delaware.

  3. Sterling shall be the surviving corporation of the Merger (in such
capacity, the "Surviving Corporation").

  4. As set forth in Section 2.05(a) of the Merger Agreement, upon the filing of
this Certificate of Merger, the Certificate of Incorporation of the Surviving
Corporation shall be amended to read in its entirety as set forth in Exhibit A
attached hereto and incorporated herein by reference.

  5. The executed Merger Agreement is on file at the principal place of business
of the Surviving Corporation, 1200 Smith Street, Suite 1900, Houston, Texas
77002-4312.

  6. A copy of the Merger Agreement will be furnished by the Surviving
Corporation, on request and without cost, to any stockholder of Acquisition or
Sterling.

     IN WITNESS WHEREOF, the Surviving Corporation has executed this Certificate
of Merger this 21 day of August, 1996.

                        STERLING CHEMICALS, INC.



                        By: /s/ J. Virgil Waggoner
                            ---------------------------------
                                J. Virgil Waggoner, President
<PAGE>
 
                                   EXHIBIT A

                    RESTATED CERTIFICATE OF INCORPORATION OF
                       STERLING CHEMICALS HOLDINGS, INC.
                       ---------------------------------



                                   (Attached)
<PAGE>

 
                     RESTATED CERTIFICATE OF INCORPORATION

                                       OF

                       STERLING CHEMICALS HOLDINGS, INC.


                                   ARTICLE I

                         Name, Registration and Purpose
                         ------------------------------

     Section 1.01.  Name.  The name of the Corporation is "Sterling Chemicals
Holdings, Inc.".

     Section 1.02.  Registered Office and Registered Agent.  The registered
office of the Corporation in the State of Delaware is located at Corporation
Trust Center, 1209 Orange Street in the City of Wilmington, County of New
Castle.  The name of the registered agent of the Corporation at such address is
The Corporation Trust Company.

     Section 1.03.  Purpose.  The purpose for which the Corporation is organized
is to engage in any lawful acts and activities for which corporations may be
organized under the General Corporation Law of the State of Delaware ("DGCL"),
and the Corporation shall have the power to perform all lawful acts and
activities.

                                   ARTICLE II

                                 Capitalization
                                 --------------

     Section 2.01.  Authorized Capital.  (a) The total number of shares of stock
that the Corporation shall have the authority to issue is 22,000,000 shares of
capital stock, consisting of (i) 2,000,000 shares of preferred stock, par value
$0.01 per share (the "Preferred Stock"), and (ii) 20,000,000 shares of common
stock, par value $0.01 per share (the "Common Stock").

     (b) Subject to the provisions of this Certificate of Incorporation and the
Preferred Stock Designation (as defined below) creating any series of Preferred
Stock, the Corporation may issue shares of its capital stock from time to time
for such consideration (not less than the par value thereof) as may be fixed by
the Board of Directors of the Corporation (the "Board of Directors"), which is
expressly authorized to fix the same in its absolute discretion subject to the
foregoing conditions.  Shares so issued for which the consideration shall have
been paid or delivered to the Corporation shall be deemed fully paid stock and
shall not be liable to any further call or assessment thereon, and the holders
of such shares shall not be liable for any further payments in respect of such
shares.

<PAGE>
 
     (c) The right to cumulate votes for the election of directors as provided
in Section 214 of the DGCL shall not be granted and is hereby expressly denied.

     Section 2.02.  Preferred Stock.  (a)  The Preferred Stock may be issued
from time to time in one or more series.  Authority is hereby expressly granted
to and vested in the Board of Directors to authorize from time to time the
issuance of Preferred Stock in one or more series.   With respect to each series
of Preferred Stock authorized by it, the Board of Directors shall be authorized
to establish by resolution or resolutions, and by filing a certificate pursuant
to the applicable law of the State of Delaware (a "Preferred Stock
Designation"), the following to the fullest extent now or hereafter permitted by
the DGCL:

   (i)  the designation of such series;

   (ii) the number of shares to constitute such series;

   (iii)  whether such series is to have voting rights (full, special or
limited) or is to be without voting rights;

   (iv) if such series is to have voting rights, whether or not such series is
to be entitled to vote as a separate class either alone or together with the
holders of the Common Stock or one or more other series of Preferred Stock;

   (v) the preferences and relative, participating, optional, conversion or
other special rights (if any) of such series and the qualifications, limitations
or restrictions (if any) with respect to such series;

   (vi) the redemption rights and price(s), if any, of such series, and whether
or not the shares of such series shall be subject to the operation of retirement
or sinking funds to be applied to the purchase or redemption of such shares for
retirement and, if such retirement or sinking funds or funds are to be
established, the periodic amount thereof and the terms and provisions relative
to the operation thereof;

   (vii)  the dividend rights and preferences (if any) of such series,
including, without limitation, (A) the rates of dividends payable thereon, (B)
the conditions upon which and the time when such dividends are payable, (C)
whether or not such dividends shall be cumulative or noncumulative and, if
cumulative, the date or dates from which such dividends shall accumulate and (D)
whether or not the payment of such dividends shall be preferred to the payment
of dividends payable on the Common Stock or any other series of Preferred Stock;

   (viii)  the preferences (if any), and the amounts thereof, which the holders
of such series shall be entitled to receive upon the voluntary or involuntary
liquidation, dissolution or winding-up of, or upon any distribution of the
assets of, the Corporation;

                                      -2-

<PAGE>
 
   (ix) whether or not the shares of such series, at the option of the
Corporation or the holders thereof or upon the happening of any specified event,
shall be convertible into or exchangeable for (A) shares of Common Stock, (B)
shares of any other series of Preferred Stock or (C) any other stock or
securities of the Corporation;

   (x) if such series is to be convertible or exchangeable, the price or prices
or ratio or ratios or rate or rates at which such conversion or exchange may be
made and the terms and conditions (if any) upon which such price or prices or
ratio or ratios or rate or rates may be adjusted; and

   (xi) such other rights, powers and preferences with respect to such series as
may to the Board of Directors seem advisable.

Any series of Preferred Stock may vary from any other series of Preferred Stock
in any or all of the foregoing respects and in any other manner.

       (b) The Board of Directors may, with respect to any existing series of
Preferred Stock but subject to the Preferred Stock Designation creating such
series, (i) increase the number of shares of Preferred Stock designated for such
series by a resolution adding to such series authorized and unissued shares of
Preferred Stock not designated for any other series or (ii) decrease the number
of shares of Preferred Stock designated for such series by a resolution
subtracting from such series shares of Preferred Stock designated for such
series (but not below the number of shares of such series then outstanding), and
the shares so subtracted shall become authorized, unissued and undesignated
shares of Preferred Stock.

       (c) No vote of the holders of the Common Stock or the Preferred Stock
shall, unless otherwise expressly provided in a Preferred Stock Designation, be
a prerequisite to the issuance of any shares of any series of the Preferred
Stock authorized by and complying with the conditions of this Certificate of
Incorporation.  Shares of any series of Preferred Stock that have been
authorized for issuance pursuant to this Certificate of Incorporation and that
have been issued and reacquired in any manner by the Corporation (including upon
conversion or exchange thereof) shall be restored to the status of authorized
and unissued shares of Preferred Stock and may be reissued as part of a new
series of Preferred Stock to be created by resolution or resolutions of the
Board of Directors and a Preferred Stock Designation as set forth above.

       Section 2.03.  Common Stock.  (a)  The holders of shares of the Common
Stock shall be entitled to vote upon all matters submitted to a vote of the
common stockholders of the Corporation and shall be entitled to one vote for
each share of the Common Stock held.

       (b) Subject to the prior rights and preferences (if any) applicable to
shares of Preferred Stock of any series, the holders of shares of the Common
Stock shall be entitled to receive such dividends (payable in cash, stock or
otherwise) as may be declared thereon by the Board of Directors at any time and
from time to time out of any funds of the Corporation legally available
therefor.

                                      -3-
<PAGE>
 
       (c) In the event of any voluntary or involuntary liquidation, dissolution
or winding-up of the Corporation, after payment or provision for payment of the
debts and other liabilities of the Corporation, and subject to the preferential
or other rights (if any) of the holders of shares of the Preferred Stock in
respect thereof, the holders of shares of the Common Stock shall be entitled to
receive all the remaining assets of the Corporation available for distribution
to its stockholders, ratably in proportion to the number of shares of the Common
Stock held by them.  For purposes of this paragraph (c), a liquidation,
dissolution or winding-up of the Corporation  shall not be deemed to be
occasioned by or to include (i) any consolidation or merger of the Corporation
with or into another corporation or other entity or (ii) a sale, lease, exchange
or conveyance of all or a part of the assets of the Corporation.

       Section 2.04.  Stock Options, Warrants, etc.  Unless otherwise expressly
prohibited in a Preferred Stock Designation creating any series of Preferred
Stock, the Corporation shall have authority to create and issue warrants, rights
and options entitling the holders thereof to purchase from the Corporation
shares of the Corporation's capital stock of any class or series or other
securities of the Corporation for such consideration and to such persons, firms
or corporations as the Board of Directors, in its sole discretion, may
determine, setting aside from the authorized but unissued stock of the
Corporation the requisite number of shares for issuance upon the exercise of
such warrants, rights or options.  Such warrants, rights and options shall be
evidenced by one or more instruments approved by the Board of Directors.  The
Board of Directors shall be empowered to set the exercise price, duration, time
for exercise and other terms of such warrants, rights or options; provided,
however, that the consideration to be received for any shares of capital stock
subject thereto shall not be less than the par value thereof.

                                  ARTICLE III

                                   Directors
                                   ---------

       Section 3.01.    Number and Term.  The number of directors of the
Corporation shall from time to time be fixed exclusively by the Board of
Directors in accordance with, and subject to the limitations set forth in, the
bylaws of the Corporation (the "Bylaws"); provided, however, that the Board of
Directors shall at all times consist of a minimum of three and a maximum of
fifteen members, subject, however, to increases above fifteen members as may
required in order to permit the holders of any series of Preferred Stock to
exercise their right (if any) to elect additional directors under specified
circumstances.  No decrease in the number of directors shall have the effect of
shortening the term of any incumbent director.  Anything in this Certificate of
Incorporation or the Bylaws to the contrary notwithstanding, each director shall
hold office until his successor is elected and qualified or until his earlier
death, resignation or removal.

       Section 3.02.  Nomination and Election.  (a) Nominations of persons for
election or reelection to the Board of Directors may be made by or at the
direction of the Board of Directors. The Bylaws may set forth procedures for the
nomination of persons for election or reelection to the Board of Directors and
only persons who are nominated in accordance with such procedures (if any)


                                      -4-
<PAGE>
 
shall be eligible for election or reelection as directors of the Corporation;
provided, however, that such procedures shall not infringe upon (i) the right of
the Board of Directors to nominate persons for election or reelection to the
Board of Directors or (ii) the rights of the holders of any series of Preferred
Stock, voting separately by class or series, to elect additional directors under
specified circumstances.

       (b) Each director shall be elected in accordance with this Certificate of
Incorporation, the Bylaws and applicable law.  Election of directors by the
Corporation's stockholders need not be by written ballot unless the Bylaws so
provide.

       Section 3.03.  Removal.  No director may be removed except by the
affirmative vote of the holders of not less than a majority in voting power of
all the outstanding shares of capital stock of the Corporation entitled to vote
generally in an election of directors, voting together as a single class. The
Board of Directors may not remove any director, and no recommendation by the
Board of Directors that a director be removed may be made to the Corporation's
stockholders unless such recommendation is set forth in a resolution adopted by
the affirmative vote of not less than two-thirds of the whole Board of
Directors.

       Section 3.04.  Vacancies.  (a) In case any vacancy shall occur on the
Board of Directors because of death, resignation or removal, such vacancy may be
filled only by a majority (or such higher percentage as may be specified in the
Bylaws) of the directors remaining in office (though less than a quorum), and
the director so appointed shall serve for the unexpired term of his predecessor
or until his successor is elected and qualified or until his earlier death,
resignation or removal.  If there are no directors then in office, an election
of directors may be held in the manner provided by applicable law.

       (b) Any newly-created directorship resulting from any increase in the
number of directors may be filled only by a majority (or such higher percentage
as may be specified in the Bylaws) of the directors then in office (though less
than a quorum).  Each director so appointed shall hold office until his
successor is elected and qualified or until his earlier death, resignation or
removal.

       (c) Except as expressly provided in this Certificate of Incorporation or
as otherwise provided by applicable law, stockholders of the Corporation shall
not have the right to fill vacancies or newly-created directorships on the Board
of Directors.

       Section 3.05.  Subject to Rights of Holders of Preferred Stock.
Notwithstanding the foregoing provisions of this Article III, if the Preferred
Stock Designation creating any series of Preferred Stock entitles the holders of
such Preferred Stock, voting separately by class or series, to elect additional
directors under specified circumstances, then all provisions of such Preferred
Stock Designation relating to the nomination, election, term of office, removal,
filling of vacancies and other features of such directorships shall, as to such
directorships, govern and control over any conflicting provisions of this
Article III.


                                      -5-
<PAGE>
 
       Section 3.06.  Limitation of Access of Stockholders to Books and Records.
In furtherance of, and not in limitation of, the powers conferred by law, the
Board of Directors is expressly authorized and empowered to determine from time
to time whether and to what extent, and at what times and places, and under what
conditions and regulations, the accounts and books of the Corporation, or any of
them, shall be open to inspection of stockholders and, except as so determined
or as expressly provided in this Certificate of Incorporation or in any
Preferred Stock Designation, no stockholder shall have any right to inspect any
account, book or document of the Corporation other than such rights as may be
conferred by applicable law.

       Section 3.07.  Limitation of Personal Liability.  (a) No person who is or
was a director of the Corporation shall be personally liable to the Corporation
or its stockholders for monetary damages for breach of fiduciary duty as a
director, except for liability (i) for any breach of the director's duty of
loyalty to the Corporation or its stockholders, (ii) for acts or omissions not
in good faith or which involve intentional misconduct or a knowing violation of
law, (iii) under Section 174 of the DGCL or (iv) for any transaction from which
the director derived an improper personal benefit.

       (b) If the DGCL is hereafter amended to authorize corporate action
further limiting or eliminating the personal liability of directors, then the
personal liability of the directors to the Corporation or its stockholders shall
be limited or eliminated to the full extent permitted by the DGCL, as so amended
from time to time.

                                   ARTICLE IV

                              Amendment of Bylaws
                              -------------------

       The Board of Directors is expressly authorized and empowered to adopt,
alter, amend or repeal the Bylaws.  Stockholders of the Corporation shall have
the power to alter, amend, expand or repeal the Bylaws but only by the
affirmative vote of the holders of not less than 66-2/3% in voting power of all
outstanding shares of capital stock of the Corporation entitled to vote
generally at an election of directors, voting together as a single class.

                                   ARTICLE V

                     Actions and Meetings of  Stockholders
                     --------------------------------------   

       Section 5.01.  No Action by Written Consent.  No action shall be taken by
the stockholders of the Corporation except at an annual or special meeting of
stockholders.  Stockholders of the Corporation may not act by written consent in
lieu of a meeting.

       Section 5.02.  Meetings.  (a) Meetings of the stockholders of the
Corporation (whether annual or special) may only be called by the Board of
Directors or such officer or officers of the Corporation as the Board of
Directors may from to time authorize to call meetings of the stockholders of the
Corporation. Stockholders of the Corporation shall not be entitled to call any


                                      -6-
<PAGE>
 
meeting of stockholders or to require the Board of Directors or any officer or
officers of the Corporation to call a meeting of stockholders except as
otherwise expressly provided by law or in the Preferred Stock Designation
creating any series of Preferred Stock.

       (b) Stockholders of the Corporation shall have the right to propose
business for consideration at any meeting of stockholders but only as may be
expressly provided in, and then only in compliance with, the Bylaws.

       (c) Business transacted at any special meeting of stockholders shall be
limited to the purposes stated in the notice or waivers of notice of such
meeting.  The person presiding at a meeting of stockholders may determine
whether business has been properly brought before the meeting and, if the facts
so warrant, such person may refuse to transact any business at such meeting
which has not been properly brought before such meeting.

       Section 5.03.  Appoint and Remove Officers, etc.  The stockholders of the
Corporation shall have no right or power to appoint or remove officers of the
Corporation nor to abrogate the power of the Board of Directors to elect and
remove officers of the Corporation.  Except as provided in Section 3.03, the
stockholders of the Corporation shall have no power to appoint or remove
directors as members of committees of the Board of Directors nor to abrogate the
power of the Board of Directors to establish one or more such committees or the
power of any such committee to exercise the powers and authority of the Board of
Directors.

                                   ARTICLE VI

                   Indemnification of Directors and Officers
                   -----------------------------------------

       The Corporation shall (i) indemnify, to the fullest extent permitted by
applicable law, each person who is or was a director or officer of the
Corporation and each person who, at the request of the Board of Directors of the
Corporation or an officer of the Corporation, is or was a director or officer of
another corporation, partnership, joint venture, trust or other enterprise, and
(ii) may indemnify each employee and agent of the Corporation and all other
persons whom the Corporation is authorized to indemnify under the provisions of
the DGCL.  Without limiting the generality or effect of the foregoing, the
Corporation may enter into one or more agreements with any person which provide
for indemnification greater or different than that provided in this Article VI.

                                  ARTICLE VII
                                        
               Election to be Governed by Section 203 of the DGCL
               --------------------------------------------------

       The Corporation hereby elects to be governed by Section 203 of the DGCL;
provided, however, that the provisions of this Article VII shall not apply to
restrict a business combination between the Corporation and an interested
stockholder (as defined in Section 203 of the DGCL) of


                                      -7-
<PAGE>
 
the Corporation if either (i) such business combination was approved by the
Board of Directors prior to the time that such stockholder became an interested
stockholder or (ii) such stockholder became an interested stockholder as a
result of, and at or prior to the effective time of, a transaction which was
approved by the Board of Directors prior to the time that such stockholder
became an interested stockholder.

                                  ARTICLE VIII
                                        
                   Amendment of Certificate of Incorporation
                   -----------------------------------------

       The Corporation reserves the right to amend, alter, change or repeal any
provisions contained in this Certificate of Incorporation or a Preferred Stock
Designation, in the manner now or hereafter prescribed by applicable law, and
all rights, preferences and privileges conferred upon stockholders, directors or
any other persons by and pursuant to this Certificate of Incorporation are
granted subject to this reservation.  Notwithstanding the foregoing or any other
provision of this Certificate of Incorporation or any provision of law that
might otherwise permit a lesser or no vote, the provisions of this Article VIII
and of Articles III, IV and V may not be repealed or amended in any respect, and
no provision inconsistent with any such provision or imposing cumulative voting
in the election of directors may be added to this Certificate of Incorporation,
unless such action is approved by the affirmative vote of the holders of not
less than 66-2/3% in voting power of all outstanding shares of capital stock of
the Corporation entitled to vote generally at an election of directors, voting
together as a single class; provided, however, that any amendment or repeal of
Section 3.07 or Article VI of this Certificate of Incorporation shall not
adversely affect any right or protection existing thereunder in respect of any
act or omission occurring prior to such amendment or repeal and, provided
further, that no Preferred Stock Designation shall be amended after the issuance
of any shares of the series of Preferred Stock created thereby, except in
accordance with the terms of such Preferred Stock Designation and the
requirements of applicable law.

                                   ARTICLE IX

                       Voting Requirements Not Exclusive
                       ---------------------------------

       The voting requirements contained in this Certificate of Incorporation
shall be in addition to the voting requirements imposed by law or by the
Preferred Stock Designation creating any series of Preferred Stock.


                                      -8-