
<PAGE>
 
                                                                     Exhibit 8.1
                                                                     -----------
                          GOODWIN, PROCTER & HOAR LLP

                               COUNSELORS AT LAW
                                 EXCHANGE PLACE
                        BOSTON, MASSACHUSETTS 02109-2881



                                January 19, 1999



Boston Properties, Inc.
8 Arlington Street
Boston, Massachusetts 02116

Ladies and Gentlemen:

     We have acted as counsel to Boston Properties, Inc., a Delaware corporation
(the "Company") in connection with the preparation of a registration statement
(the "Registration Statement") filed with the Securities and Exchange Commission
on Form S-3, as amended, relating to the issuance of up to 1,471,456 shares (the
"Redemption Shares") of common stock, par value $.01 per share ("Common Stock"),
of the Company.  The Redemption Shares may be issued by the Company if and to
the extent that common units of limited partnership interest ("Redemption
Units") in Boston Properties Limited Partnership, a Delaware limited partnership
(the "Operating Partnership"), that were originally issued to John F. Griffin,
George G. Mulligan, Barbara Huppe, Barry M. Fitzpatrick, David E. Schutt,
Charles A. Salcetti, Nancy M. Griffin, Caroline A. Griffith, David M. Whitmer,
George H. Beuchert, III, Cynthia A. Benedetti, The Donald  N. Coupard Revocable
Trust, The Patricia E. Coupard Revocable Trust, Tech Park 270 Phase III Limited
Partnership, Tech Park 270 Limited Partnership, Decoverly Two Limited
Partnership, Reston Town Center Office Park Phase One Limited Partnership, MGA
Virginia 85-1 Limited Partnership, MGA Virginia 86-2 Limited Partnership,
Decoverly Four Limited Partnership, Decoverly Five Limited Partnership,
Decoverly Six Limited Partnership, Decoverly Seven Limited Partnership, Reston
Corporate Center Limited Partnership, and MGA Virginia 96-1 Limited Partnership,
in connection with the acquisition of properties in Maryland and Virginia on
February 1, 1998, are presented to the Operating Partnership for redemption
(which may occur at any time on or after February 11, 1999) and the Company
exercises its right under the partnership agreement of the Operating Partnership
to acquire such Redemption Units in exchange for shares of Common Stock.  You
have requested our opinion on certain federal income tax matters in connection
with the issuance.
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Boston Properties, Inc.
January 19, 1999
Page 2


     Capitalized terms not defined herein shall have the same meaning as in the
Registration Statement.

     In rendering the following opinion, we have examined the Amended and
Restated Certificate of Incorporation (the "Articles") and Bylaws of the
Company, and such other records, certificates and documents as we have deemed
necessary or appropriate for purposes of rendering the opinion set forth herein
(collectively, the "Documents").  We have reviewed the investment activities,
operations and governance of the Company and its subsidiaries.  We have relied
upon representations of duly appointed officers of the Company and the Operating
Partnership (including without limitation, representations contained in a letter
dated January 19, 1999 (the "Officer's Certificate")), principally relating to
the Company's organization and operations.  We assume that each such
representation is and will be true, correct and complete and that all
representations that speak in the future, or to the intention, or to the best of
the belief and knowledge of any person(s) or party(ies) are and will be true,
correct and complete as if made without such qualification.  Nothing has come to
our attention which would cause us to believe that any of such representations
are untrue, incorrect or incomplete.  We assume that the Company will be
operated in accordance with the applicable laws and the terms and conditions of
applicable documents.  We have also reviewed the Registration Statement, the
Amended and Restated Agreement of Limited Partnership of the Operating
Partnership, and such other documents as we have deemed appropriate.  In
addition, we have relied upon certain additional facts and assumptions described
below.

     In rendering the opinion set forth herein, we have assumed (i) the
genuineness of all signatures on documents we have examined, (ii) the
authenticity of all documents submitted to us as originals, (iii) the conformity
to the original documents of all documents submitted to us as copies, (iv) the
conformity of final documents to all documents submitted to us as drafts, (v)
the authority and capacity of the individual or individuals who executed any
such documents on behalf of any person, (vi) the accuracy and completeness of
all records made available to us, and (vii) the factual accuracy of all
representations, warranties and other statements made by all parties.  In
addition, we assume that all interests in the Operating Partnership have been
and will be issued in a transaction (or transactions) that are not required to
be registered under the Securities Act of 1933 and that no interest in the
Operating Partnership offered for sale outside the United States would have been
required to be registered under the Securities Act of 1933 if such interest had
been offered for sale within the United States.  We have further assumed that
during its short 1997 taxable year ending December 31, 1997 and subsequent
taxable years, the Company has operated and will operate in such a manner that
has made and will make the representations contained in the Officer's
Certificate true for all such years, and that the Company and its subsidiaries
will not make any amendments to its organizational documents 
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Boston Properties, Inc.
January 19, 1999
Page 3


after the date of this opinion that would affect the Company's qualification as
a real estate investment trust for any taxable year. For purposes of our
opinion, we have made no independent investigation of the facts contained in the
documents and assumptions set forth above, the representations set forth in the
Officer's Certificate, or the Registration Statement. No facts have come to our
attention, however, that would cause us to question the accuracy and
completeness of such facts or documents in a material way.

     The discussion and conclusion set forth below are based upon the Code, the
Income Tax Regulations and Procedure and Administration Regulations promulgated
thereunder and existing administrative and judicial interpretation thereof, all
of which are subject to change.  No assurance can therefore be given that the
federal income tax consequences described below will not be altered in the
future.  Based on the documents and assumptions set forth above, the
representations set forth in the Officer's Certificate and provided that the
Company continues to meet the applicable asset composition, source of income,
shareholder diversification, distribution, and other requirements of the Code
necessary for a corporation to qualify as a real estate investment trust, we are
of the opinion that:

     (1) Commencing with the Company's initial taxable year ended December 31,
1997, the Company has been operated and organized in conformity with the
requirements for qualification as a "real estate investment trust" under the
Code, and its method of operation, as described in the Officers Certificate,
will enable it to continue to meet the requirements for qualification as a "real
estate investment trust" under the Code, and

     (2) The information in the Registration Statement under the caption
"Certain Federal Income Tax Considerations" to the extent that it constitutes
matters of law or legal conclusions, have been reviewed by us and is correct in
all material respects, and our opinion set forth in such discussion is
confirmed.

     We will not review on a continuing basis the Company's compliance with the
documents or assumptions set forth above, or the representations set forth in
the Officer's Certificate. Accordingly, no assurance can be given that the
actual results of the Company's operations for any given taxable year will
satisfy the requirements for qualification and taxation as a real estate
investment trust under the Code.  The ability of the Company to continue to meet
the requirements for qualification and taxation as a real estate investment
trust will be dependent upon the Company's ability to continue to meet in each
year the applicable asset composition, source of income, shareholder
diversification, distribution, and other requirements of the Code necessary for
a corporation to qualify as a real estate investment trust.  The foregoing
opinions are limited to the federal income tax matters addressed herein, and no
other opinions are rendered 
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Boston Properties, Inc.
January 19, 1999
Page 4

with respect to other federal tax matters or to any issues arising out of the
tax laws of any state or locality. We express no opinion with respect to the
transactions described herein other than those expressly set forth herein. You
should recognize that our opinion is not binding on the Internal Revenue Service
and that the Internal Revenue Service may disagree with the opinion contained
herein. Although we believe that our opinion will be sustained if challenged,
there is no guarantee that this will be the case. Except as specifically
discussed above, the opinion expressed herein is based upon the laws that
currently exist. Consequently, future changes in the law may cause the federal
income tax treatment of the transactions herein to be materially and adversely
different from that described above. This opinion may be relied on solely by you
in connection with the issuance.

     We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement.  We also consent to the references to Goodwin, Procter &
Hoar,  LLP under the caption "Certain Federal Income Tax Considerations" in the
Registration Statement.  In giving this consent, we do not admit that we are in
the category of persons whose consent is required by Section 7 of the Securities
Act of 1933, as amended, or the rules and regulations promulgated thereunder by
the Securities and Exchange Commission.

                                    Very truly yours,


                                    /s/ Goodwin, Procter & Hoar  LLP

                                    Goodwin, Procter & Hoar  LLP
