EXHIBIT 5
                                SULLIVAN & WORCESTER LLP
                                ONE POST OFFICE SQUARE
                              BOSTON, MASSACHUSETTS 02109
                                    (617) 338-2800
                                 FAX NO. 617-338-2880
    IN WASHINGTON, D.C.                                    IN NEW YORK CITY
1025 CONNECTICUT AVENUE, N.W.                              767 THIRD AVENUE
   WASHINGTON, D.C. 20036                              NEW YORK, NEW YORK 10017
      (202) 775-8190                                         (212) 486-8200
   FAX NO. 202-293-2275                                    FAX NO. 212-758-2151



    

                                                             October 24, 1997



SIS Bancorp, Inc.
1441 Main Street
Springfield, Massachusetts 01102

         Re:      Registration Statement on Form S-3
                  146,400 shares of Common Stock, par value $.01 per share

Gentlemen:

         The  following  opinion  is  furnished  to you in  connection  with the
registration  by SIS Bancorp,  Inc.,  a Delaware  corporation  (the  "Company"),
pursuant to a registration statement on Form S-3 (the "Registration  Statement")
under the Securities Act of 1933, as amended (the "Act"), of 146,400 shares (the
"Shares") of Common Stock, par value $.01 par value (the "Common  Stock").  Some
or all of the  Shares  may be offered  by the  Company  in  connection  with the
Agreement  and Plan of  Reorganization  dated as of August 18, 1997  between the
Company and Glastonbury Bank & Trust Company and the  transactions  contemplated
thereby,  which is the subject of a separate Registration  Statement on Form S-4
filed this date with the Securities and Exchange Commission.

         We have  acted  as  counsel  to the  Company  in  connection  with  the
Registration Statement,  and we have examined originals or copies,  certified or
otherwise  identified to our satisfaction,  of the Registration  Statement,  the
Certificate of  Incorporation  of the Company as presently in effect,  corporate
records,  certificates and statements of officers and accountants of the Company
and of  public  officials,  and  such  other  documents  as we  have  considered
necessary in order to furnish the opinion  hereinafter  set forth. We express no
opinion  herein as to any laws other  than the  General  Corporation  Law of the
State of Delaware.


SIS Bancorp, Inc.
October 24, 1997
Page 2

         Based on and subject to the foregoing,  we are of the opinion that: (a)
the Shares have been duly and validly  authorized  by the  Company;  and (b) all
necessary  action on the part of the Company in connection  with the issuance of
said Shares has been taken and, upon  issuance of the Shares in accordance  with
the terms of the Prospectus contained in the Registration Statement, said Shares
will be validly issued, fully paid and nonassessable by the Company.

         We hereby  consent to the  filing of this  opinion as an exhibit to the
Registration  Statement  and to the reference to our firm made therein under the
caption "Legal Matters." In giving such consent, we do not thereby admit that we
come within the category of persons whose consent is required under Section 7 of
the Act or the rules and  regulations of the Securities and Exchange  Commission
promulgated thereunder.

                                               Very truly yours,

                                               /s/SULLIVAN & WORCESTER LLP

                                               SULLIVAN & WORCESTER LLP