NUTTER, MCCLENNEN & FISH, LLP

                                ATTORNEYS AT LAW

                            ONE INTERNATIONAL PLACE
                        BOSTON, MASSACHUSETTS 02110-2699


                TELEPHONE: 617 439-2000 FACSIMILE: 617 973-9748


CAPE COD OFFICE                                              DIRECT DIAL NUMBER
HYANNIS, MASSACHUSETTS



                                                                 (617) 439-2459
                                                                 E-MAIL ADDRESS
                                                                 kmk@nutter.com
                                                                 --------------


                               September 12, 2000



Mayer, Brown & Platt
1675 Broadway
New York, NY 10019-5820


Dear Sirs:

     We understand that the trustees of Morgan Stanley Dean Witter Tax-Exempt
Securities Trust, a Massachusetts business trust (the "Trust"), intend, on or
about September 13, 2000, to cause to be filed on behalf of the Trust a
Registration Statement on Form N-14 (the "Registration Statement") in connection
with the acquisition (the "Acquisition") by the Trust of substantially all the
assets of Morgan Stanley Dean Witter Municipal Income Trust III ("MIT III"), in
exchange for shares of beneficial interest of the Trust (the "Shares"), and the
assumption by the Trust of certain stated liabilities of MIT III pursuant to an
Agreement and Plan of Reorganization dated as of August 24, 2000 between the
Trust and MIT III (the "Agreement"). We further understand that the Shares will
be issued pursuant to the Agreement.

     You have requested that we act as special counsel to the Trust with respect
to the laws of the Commonwealth of Massachusetts on certain specified matters,
and in such capacity we are furnishing you with this opinion. You have not asked
for, and we do not offer, an opinion on any other matter or transaction related
to the Trust, MIT III, the Acquisition, the Agreement or any matter related
thereto, except as specifically set forth below.

     The Trust is a business trust created under an Agreement and Declaration of
Trust finally executed, delivered and filed in Boston, Massachusetts on April 6,
1987 (as amended, the "Trust Agreement"). The Trustees of the Trust (as defined
in the Trust Agreement) (the "Trustees") have the powers set forth in the Trust
Agreement, subject to the terms, provisions and conditions provided therein.

     In connection with our opinions delivered herein, we have examined the
following items some of which have been provided to us by, or on behalf of, you:
(i) a copy of the Agreement in the form to be executed by the Trust and MIT III;
(ii) a copy of the Trust Agreement; (iii) a copy of the Amended and Restated
By-laws of the Trust effective as of May 1, 1999; (iv) a Certificate of Legal
Existence for the Trust provided by the Secretary of State of the Commonwealth
of Massachusetts dated September 11, 2000; and (v) copies of the Registration
Statement on Form N-14 to be filed by the Trust and the Trust's current
Prospectus and Statement of Additional Information.



     In rendering this opinion we have assumed, without independent
verification, (i) the due authority of all individuals signing in representative
capacities and the genuineness of signatures, (ii) the authenticity,
completeness and continued effectiveness of all documents or copies furnished to
us, (iii) that resolutions approving the Registration Statement, the Acquisition
and the Agreement have been duly adopted by the Trustees, (iv) that no
amendments, agreements, resolutions or actions have been approved, executed or
adopted which would limit, supersede or modify the items described above, and
(v) that the by-laws filed as an exhibit to the Registration Statement have been
duly adopted by the Trustees. We have also examined such questions of law as we
have concluded necessary or appropriate for purposes of the opinions expressed
below. Where documents are referred to in resolutions approved by the Trustees,
or in the Registration Statement, we assume such documents are the same as in
the most recent form provided to us, whether as an exhibit to the Registration
Statement, or otherwise. When any opinion set forth below relates to the
existence or standing of the Trust, such opinion is based entirely upon and is
limited by the items referred to above. We understand that the foregoing
assumptions, limitations and qualifications are acceptable to you.

     Based upon the foregoing, and with respect to Massachusetts law only
(except that no opinion is herein expressed with respect to compliance with the
Massachusetts Uniform Securities Act), to the extent that Massachusetts law may
be applicable, and without reference to the laws of any of the other several
states or of the United States of America, including State and Federal
securities laws, we are of the opinion that:

         1. The Trust is a business trust with transferable shares, organized in
     compliance with the requirements of The Commonwealth of Massachusetts, and
     the Trust Agreement is legal and valid.

         2. The Shares to be issued as described in the Registration Statement,
     including any exhibits thereto, have been duly authorized and, assuming
     receipt of the consideration to be paid therefor, upon delivery as provided
     in the Agreement, will be validly issued, fully paid and nonassessable
     (except for the potential liability of shareholders described in the
     Trust's current Statement of Additional Information dated February 24,
     2000, under the caption "Capital Stock and Other Securities").

     We understand that you will rely on this opinion solely in connection with
your opinion to be filed with the Securities and Exchange Commission as an
exhibit to the Registration Statement. We hereby consent to such use of this
opinion and we also consent to the filing of said opinion with the Securities
and Exchange Commission. In so consenting, we do not thereby admit to be within
the category of persons whose consent is required under Section 7 of the
Securities Act of 1933, as amended, or the rules and regulations of the
Securities and Exchange Commission thereunder.


                                        Very truly yours,

                                        /s/ Nutter, McClennen & Fish, LLP
                                        ---------------------------------------
                                            NUTTER, MCCLENNEN & FISH, LLP


JFM/KMK

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