Exhibit 8.1

                         Tax Opinion of Foley Hoag LLP

                                                                   Boston Office
January 31, 2005                                                   617.832.1000

Benjamin Franklin Bancorp, M.H.C.
58 Main Street
P.O. Box 309
Franklin, Massachusetts  02038-0927

      Re:   Proposed Merger of Benjamin Franklin Savings Bank and Chart Bank

Ladies and Gentlemen:

      We have acted as counsel to Benjamin Franklin Bancorp, M.H.C., a
Massachusetts-chartered mutual holding company ("Bancorp"), in connection with
the proposed merger (the "Merger") of Chart Bank, a Massachusetts-chartered
stock co-operative bank ("Chart"), with and into Benjamin Franklin Savings Bank,
a Massachusetts-chartered stock savings bank ("Bancorp Bank") and wholly-owned
subsidiary of Bancorp, in accordance with the Agreement and Plan of Merger by
and among Bancorp, Bancorp Bank, and Chart, dated as of September 1, 2004 (the
"Agreement"). Capitalized terms used but not defined herein shall have the
respective meanings ascribed to them in the Agreement.

      We have examined the law and such papers, including the Agreement, as
deemed necessary to render the opinion expressed below. As to questions of fact
material to our opinion we have relied on representations of Bancorp and Chart
contained in letters addressed to us, without undertaking to verify the same by
independent investigation; upon representations set forth in the Agreement
(including the Exhibits); and upon such other documents pertaining to the Merger
as we have deemed appropriate and necessary.

      In our examination we have assumed that (i) the Merger will be consummated
in accordance with the terms of the Agreement; (ii) each entity that is a party
to any of the documents (the "Documents") described in the preceding paragraphs
has been duly organized under the laws of its state or country of organization,
is validly existing and in good standing under such laws, and is duly qualified
and in good standing in each jurisdiction in which it is required to be
qualified to engage in the transactions



Benjamin Franklin Bancorp, M.H.C.
January 31, 2005
Page 2

contemplated by the Documents; (iii) each such entity has full power, authority,
capacity and legal right to enter into and perform the terms of the Documents
and the transactions contemplated thereby; (iv) the copies or originals of the
Documents furnished to us are authentic (if originals) or accurate (if copies),
those that are contracts or instruments are enforceable and effective in
accordance with their terms against all parties thereto, and



Benjamin Franklin Bancorp, M.H.C.
January 31, 2005
Page 3

all signatures are genuine; (v) any representations made in the Documents are,
and will continue to be, true and complete, and no default exists under any of
the Documents; (vi) the business and affairs of each of the entities that is a
party to any of the Documents will be conducted in accordance with the Documents
and all relevant laws; (vii) no actions will be taken, no change in any of the
Documents will occur, and no other events will occur, after the date hereof,
that would have the effect of altering the facts, Documents or assumptions upon
which this opinion is based; and (viii) the business reasons for the Merger will
constitute a valid business purpose, within the meaning of Treasury Regulation
section 1.368-1(b) and (c), for the Merger.

      The opinion rendered herein is based upon the provisions of the Internal
Revenue Code of 1986, as amended (the "Code"), Treasury Department temporary and
final regulations, judicial decisions, and rulings and administrative
interpretations of the Internal Revenue Service, as each of the foregoing exists
on the date hereof. The opinion rendered below is not binding on the Internal
Revenue Service or a court of law, and no assurance can be given that
legislative or administrative action or judicial decisions that differ from the
opinion rendered below will not be forthcoming. Any such differences could be
retroactive to transactions or business operations prior to such action or
decisions.

      We express no opinion as to the federal income tax consequences other than
that described below, if any, or as to the effect of the Merger on other
transactions, or as to any state, local or foreign income or other tax
consequences with respect to the Merger.

      Based on the foregoing, we are of opinion, as of the date hereof and under
existing law, that the Merger will constitute a reorganization within the
meaning of section 368(a) of the Code.

      We undertake no responsibility to update or supplement our opinion. We are
furnishing this letter to you solely for the purpose of satisfying Section
8.1(h) of the Agreement and in support of the information set forth under the
heading "Material Federal Income Tax Consequences of the Merger" in the
Registration Statement. We hereby consent to the filing of this opinion with the
Securities and Exchange Commission as an exhibit to the Registration Statement
and to the reference to our firm under such heading in the Registration
Statement. In giving such consent, we do not thereby admit that we are in the
category of persons whose consent is required under Section 7 of the Securities
Act of 1933, as amended. This letter is not to be used, circulated, quoted, or
otherwise referred to for any other purpose without our prior written consent.
For purposes only of compliance with Treasury Regulation Section
1.6011-4T(b)(3)(i), we hereby agree that you (and each of your employees,
representatives, or other agents) may disclose to any and all persons,



Benjamin Franklin Bancorp, M.H.C.
January 31, 2005
Page 4

without limitation of any kind, the tax treatment and tax structure of the
Merger and all materials of any kind, including this opinion letter any other
tax analyses, that we have provided to you relating to such tax treatment and
tax structure; provided, however, that any such information relating to the tax
treatment or tax structure must be kept confidential to the extent necessary to
comply with applicable securities laws.

                                                        Very truly yours,

                                                        Foley Hoag LLP

                                                       By: Richard Schaul-Yoder
                                                           --------------------
                                                                A Partner