November 24, 1997
  PaineWebber Incorporated
  1200 Harbor Blvd.
  Weehawken, New Jersey 07087
  Ladies and Gentlemen:
  We have served as counsel for PaineWebber Incorporated as
  sponsor and depositor (the "Depositor") of The Municipal Bond
  Trust, Series 231 (hereinafter referred to as the "Trust"). The
  Depositor seeks by means of Post-Effective Amendment No. 6 to
  register for reoffering 4,097 Units acquired by the Depositor in the
  secondary market (hereinafter referred to as the "Units").
  In this regard, we have examined executed originals or copies of the
  following:
  (a)  The Restated Certificate of Incorporation, as amended, and the
       By-Laws of the Depositor, as amended;
  (b)  Resolutions of the Board of Directors of the Depositor adopted on
       December 3, 1971 relating to the Trust and the sale of the Units;
  (c)  Resolutions of the Executive Committee of the Depositor adopted
       on September 24, 1984;
  (d)  Powers of Attorney referred to in the Amendment;
  (e)  Post-Effective Amendment No. 6 to the Registration Statement on
       Form S-6 (File No. 33-38089) to be filed with the Securities and
       Exchange Commission (the "Commission") in accordance with
       the Securities Act of 1933, as amended, and the rules and
       regulations of the Commission promulgated thereunder
       (collectively, the "1933 Act") proposed to be filed on or about the
       date hereof (the "Amendment");
  (f)  The Notification of Registration of the Trust filed with the
       Commission under the Investment Company Act of 1940, as
       amended (collectively, the "1940 Act") on Form N-8A, as
       amended;
  (g)  The registration of the Trust filed with the Commission under the
       1940 Act on Form N-8B-2 (File No. 811-2599), as amended;
  (h)  The prospectus included in the Amendment (the "Prospectus");
  (i)  The Standard Terms and Conditions of the Trust dated as of
       August 10, 1990, as amended, among the Depositor, Investors
       Bank & Trust Company and the First National Bank of Chicago
       (the "Co-Trustees"), as successor Co-Trustee, and Standard &
       Poor's Corporation and Kenny Information Systems, a division of
       J.J. Kenny Co., Inc. (the "Evaluator") (the "Standard Terms");
  (j)  The Trust Indenture dated as of the Initial Date of Deposit, among
       the Depositor, the Co-Trustees and the Evaluator (the "Trust
       Indenture" and, collectively with the Standard Terms, the
       "Indenture and Agreement");
  (k)  The form of certificate of ownership for units (the "Certificate") to
       be issued under the Indenture and Agreement; and
  (l)  Such other pertinent records and documents as we have deemed
       necessary.
       With your permission, in such examination, we have assumed
  the following: (a) the authenticity of original documents and the
  genuineness of all signatures; (b) the conformity to the originals of
  all documents submitted to us as copies; (c) the truth, accuracy,
  and completeness of the information, representations, and warranties
  contained in the records, documents, instruments and certificates we
  have reviewed; (d) except as specifically covered in the opinions set
  forth below, the due authorization, execution, and delivery on behalf
  of the respective parties thereto of documents referred to herein and
  the legal, valid, and binding effect thereof on such parties; and (e)
  the absence of any evidence extrinsic to the provisions of the written
  agreement(s) between the parties that the parties intended a
  meaning contrary to that expressed by those provisions. However,
  we have not examined the securities deposited pursuant to the
  Indenture and Agreement (the "Securities") nor the contracts for the
  Securities.
       We express no opinion as to matters of law in jurisdictions other
  than the State of New York (except "Blue Sky" laws) and the federal laws
  of the United States, except to the extent necessary to render the 
  opinion as to the Depositor in paragraph (i) below with respect to 
  Delaware law.  As you know we are not licensed to practice law in the 
  State of Delaware, and our opinion in paragraph (i) and (iii) as to 
  Delaware law is based solely on review of the official statutes of the 
  State of Delaware.
       Based upon such examination, and having regard for legal
  considerations which we deem relevant, we are of the opinion that:
  (i)  The Depositor is a corporation duly organized, validly existing, and
       in good standing under the laws of the State of Delaware with full
       corporate power to conduct its business as described in the
       Prospectus;
  (ii) The Depositor is duly qualified as a foreign corporation and is in
       good standing as such within the State of New York;
  (iii)The terms and provisions of the Units conform in all material
       respects to the description thereof contained in the Prospectus;
  (iv) The consummation of the transactions contemplated under the
       Indenture and Agreement and the fulfillment of the terms thereof
       will not be in violation of the Depositor's Restated Certificate of
       Incorporation, as amended, or By-Laws, as amended and will not
       conflict with any applicable laws or regulations applicable to the
       Depositor in effect on the date hereof; and
  (v)  The Certificates to be issued by the Trust, when duly executed by
       the Depositor and the Trustee in accordance with the Indenture
       and Agreement, upon delivery against payment therefor as
       described in the Prospectus will constitute fractional undivided
       interests in the Trust enforceable against the Trust in accordance
       with their terms, will be entitled to the benefits of the Indenture
       and Agreement and will be fully paid and non-assessable.
  Our opinion that any document is valid, binding, or enforceable in
  accordance with its terms is qualified as to:
  (a)  limitations imposed by bankruptcy, insolvency, reorganization,
       arrangement, fraudulent conveyance, moratorium, or other laws
       relating to or affecting the enforcement of creditors' rights
       generally;
  (b)  rights to indemnification and contribution which may be limited by
       applicable law or equitable principles; and
  (c)  general principles of equity, regardless of whether such
       enforceability is considered in a proceeding in equity or at law.
       We hereby represent that the Amendment contains no disclosure
  which would render it ineligible to become effective immediately
  upon filing pursuant to paragraph (b) of Rule 485 of the
  Commission.
       We hereby consent to the filing of this opinion as an exhibit to
  the Amendment and to the use of our name wherever it appears in
  the Amendment and the Prospectus.
  Very truly yours,
  /s/ CARTER, LEDYARD & MILBURN