September 13, 2000 Morgan Stanley Dean Witter Tax-Exempt Securities Trust Two World Trade Center New York, New York 10048 Ladies and Gentlemen: This opinion is being furnished to Morgan Stanley Dean Witter Tax-Exempt Securities Trust, a Massachusetts business trust ("Tax-Exempt"), in connection with the Registration Statement on Form N-14 (the "Registration Statement") under the Securities Act of 1933, as amended (the "1933 Act"), to be filed by Tax-Exempt in connection with the acquisition by Tax-Exempt, of substantially all the assets of Morgan Stanley Dean Witter Municipal Income Trust III, a Massachusetts business trust ("MIT III"), in exchange for shares of beneficial interest of Tax-Exempt ("Shares") and the assumption by Tax-Exempt of certain stated liabilities of MIT III pursuant to an Agreement and Plan of Reorganization dated as of August 24, 2000 (the "Reorganization Agreement"). We have examined such statutes, regulations, corporate records and other documents and reviewed such questions of law as we deemed necessary or appropriate for the purposes of this opinion. As to matters of Massachusetts law contained in this opinion, we have relied upon the opinion of Nutter, McClennen & Fish LLP, dated September 12, 2000. Based upon the foregoing, we are of the opinion that the Shares when issued, as described in the Reorganization Agreement, will be duly authorized and, assuming receipt of the consideration to be paid therefor, upon delivery as provided in the Reorganization Agreement, will be legally issued, fully paid and non-assessable (except for the potential liability of shareholders described in Tax-Exempt's Statement of Additional Information dated February, 24, 2000 under the caption "Capital Stock And Other Securities"). We hereby consent to the filing of this opinion as an exhibit to the Registration Statement and to the reference to us under the caption "Legal Matters" in the Prospectus forming a part of the Registration Statement. We do not thereby admit that we are within the category of persons whose consent is required under Section 7 of the 1933 Act or the rules and regulations of the Securities and Exchange Commission thereunder. Very truly yours, /s/ Mayer, Brown & Platt --------------------- Mayer, Brown & Platt