<PAGE>
 
                                                                     Exhibit 8.1


                       [Mayer, Brown & Platt letterhead]



                               February 1, 1999



Cabot Industrial Trust
Two Center Plaza, Suite 200
Boston, Massachusetts 02108

     RE:  Partnership Classification; Status as a Real Estate Investment Trust
     ("REIT"); Information in the Prospectus under "FEDERAL INCOME TAX
     CONSEQUENCES"
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Ladies and Gentlemen:

     In connection with the Registration Statement on Form S-3 to be filed with
the Securities and Exchange Commission on February 1, 1999 (the "Registration
Statement") relating to the offering of Offered Securities/1/ in Cabot
Industrial Trust, a Maryland real estate investment trust (the "Company") and
Cabot Industrial Properties, L.P., a Delaware limited partnership (the
"Operating Partnership"), you have requested our opinions concerning (i) the
treatment of the Operating Partnership as a partnership for Federal income tax
purposes, and not as an association taxable as a corporation; (ii) the
qualification and taxation for Federal income tax purposes of the Company as a
REIT; and (iii) the information in the Prospectus under the heading "FEDERAL
INCOME TAX CONSEQUENCES."

     In formulating our opinions, we have reviewed and relied upon (i) the
Operating Partnership Agreement, the Declaration of Trust and Bylaws of the
Company, the Certificate of Incorporation and the Bylaws of the Management
Company (collectively, the "Organizational Documents"), (ii) the Prospectus and
(iii) such other documents and information provided by you, and such applicable
provisions of law as we have considered necessary for purposes of the opinions
expressed herein.

     In addition, we have relied upon the Company's certificate (the "Officer's
Certificate"), executed by a duly appointed officer of the Company, setting
forth certain representations relating to the organization and the actual and
the proposed method of operation of the 





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/1/  Unless otherwise specifically defined herein, all capitalized terms have
the meaning assigned to them in the Registration Statement.
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Cabot Industrial Trust
February 1, 1999
Page 2


Company, the Operating Partnership, and the Management Company. For purposes of
our opinions, we have not made an independent investigation of the facts and
representations set forth in the Officer's Certificate or in the Organizational
Documents or any of the information set forth in the Registration Statement. We
have, consequently, relied upon your representations that the information
presented in such documents accurately and completely describes all material
facts. No facts have come to our attention, however, that would cause us to
question the accuracy or completeness of such facts or documents in any material
respect.

     In rendering these opinions, we have assumed (i) the Company, the Operating
Partnership, and the Management Company will each be operated in the manner
described in the applicable Organizational Documents and in the Prospectus, and
that all terms and provisions of such agreements and documents will be complied
with by all parties thereto; and (ii) each partner in the Operating Partnership
has been motivated in acquiring its partnership interest by its anticipation of
economic rewards apart from tax considerations.

     The opinions expressed herein are based on the Code, the Treasury
regulations promulgated thereunder, and interpretations of the Code and such
regulations by the courts and the Internal Revenue Service, all as they are in
effect and exist at the date of this letter.  It should be noted that statutes,
regulations, judicial decisions, and administrative interpretations are subject
to change at any time and, in some circumstances, with retroactive effect.  A
material change that is made after the date hereof in any of the foregoing bases
for our opinions could adversely affect our conclusions.  Other than as
expressly stated below, we express no opinion on any issue relating to the
Operating Partnership, the Company or to any investment therein.

     Based upon and subject to the foregoing, it is our opinion that:

     1.   The Operating Partnership will be treated, for Federal income tax
purposes, as a partnership, and not as an association taxable as a corporation.

     2.   Beginning with the Company's taxable year ended December 31, 1998, and
assuming that the actions contemplated in the Prospectus are completed in a
timely fashion, the Company has been organized in conformity with the
requirements for qualification as a REIT under the Code, and the Company's
actual and proposed method of operation, as described in the Prospectus and as
represented in the Officer's Certificate, will enable it to satisfy the
requirements for qualification as a REIT.

     3.   The information in the Prospectus under the heading "FEDERAL INCOME
TAX CONSEQUENCES," to the extent that it constitutes matters of law, summaries
of legal matters, or legal conclusions, has been reviewed by us and is correct
in all material respects.
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Cabot Industrial Trust
February 1, 1999
Page 3




     We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to the use of the name of our firm therein.

                                    Very truly yours,


                                    MAYER, BROWN & PLATT