Exhibit 8.1



Internet ID: @thacherproffitt.com


                                                            , 2000



Cambridgeport Mutual Holding Company
Cambridgeport Bank
689 Massachusetts Avenue
Cambridge, Massachusetts 02139

Dear Ladies and Gentlemen:

     You have requested our opinion regarding certain federal income tax
consequences of the proposed conversion (the "Conversion"), more fully described
below, pursuant to which Cambridgeport Mutual Holding Company ("MHC") will
convert from a Massachusetts-chartered mutual holding company to a stock-form
institution under the name Port Financial Corp. pursuant to the Cambridgeport
Mutual Holding Company Plan of Conversion from Mutual Holding Company to Stock
Holding Company and Stock Issuance adopted by the Board of Trustees of MHC on
October 19, 1999 (the "Plan").  The Conversion and its component and related
transactions are governed by the Plan and are described in the Prospectus (the
"Prospectus") filed with the Securities and Exchange Commission in connection
with the Conversion and proposed sale of MHC common stock.  We are rendering
this opinion pursuant to Section 3.1 of the Plan. All capitalized terms used but
not defined in this letter shall have the meanings assigned to them in the Plan
or Prospectus.

     In connection with the opinions expressed below, we have examined and
relied upon originals, or copies certified or otherwise identified to our
satisfaction, of the Plan and the Prospectus and of such corporate records of
MHC and of the Bank as we have deemed appropriate.  We have also relied upon,
without independent verification, the representations of the Bank contained in
its letter to us dated ________, 2000.  We have assumed that such
representations are true and that MHC and the Bank will act in accordance with
the Plan. In addition, we have made such investigations of law as we have deemed
appropriate to form a basis for the opinions expressed below.


Cambridgeport Mutual Holding Company
Cambridgeport Bank
____________, 2000                                                      Page 2


     Based on and subject to the foregoing, it is our opinion that for federal
income tax purposes, under current law, for federal income tax purposes:

     (1) the Conversion will constitute a reorganization under Section
         368(a)(1)(F) of the Code;

     (2) none of the Bank, the MHC or Port Financial Corp. will recognize gain
         or loss as a result of the Conversion; and

     (3) Eligible Account Holders and Supplemental Eligible Account Holders will
         not recognize gain or loss upon their receipt of nontransferable
         Subscription Rights to purchase shares of Port Financial Corp.,
         provided the amount to be paid for such shares is equal to fair market
         value of such shares.

     In rendering our opinion in (iii), above, we have relied, without
independent verification, on the opinion of RP Financial, LC. that the
nontransferable Subscription Rights have no value.

     This opinion is given solely for the benefit of the Bank, MHC, Port
Financial Corp., Eligible Account Holders and Supplemental Eligible Account
Holders, and may not be relied upon by any other party or entity or referred to
in any document without our express written consent. We consent to the filing of
this opinion as an exhibit to the Application for Conversion filed with the
Division of Banks of the Commonwealth of Massachusetts in accordance with
Chapter 167H of the Massachusetts General Laws, as a supporting document to the
Application to the Board of Governors of the Federal Reserve System on Form FRY-
3 and as an exhibit to the Registration Statement on Form S-1 filed with the
Securities and Exchange Commission.

                                  Very truly yours,

                                  THACHER PROFFITT & WOOD


                                  By: