SULLIVAN & WORCESTER LLP
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     IN WASHINGTON, D.C.                                   IN NEW YORK CITY
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    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



                                                             September 4, 1996



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

         Re:  Registration Statement on Form S-8 of 868,650 shares of Common
              Stock, par value $.01 per share

Ladies and Gentlemen:

         In connection with the  registration  under the Securities Act of 1933,
as amended (the "Act"), by SIS Bancorp,  Inc., a Massachusetts  corporation (the
"Company"), of 868,650 shares (the "Registered Shares") of its Common Stock, par
value $.01 per share ("Common Stock"),  all of which Registered Shares are to be
offered by the Company,  the  following  opinion is furnished to you to be filed
with the Securities and Exchange  Commission (the  "Commission") as Exhibit 5 to
the Company's registration statement on Form S-8 (the "Registration  Statement")
under  the  Act.  The  Registered  Shares  are to be  offered  on a  delayed  or
continuous  basis pursuant to Rule 415 under the Act in connection  with options
granted  under the Company's  Director  Stock Option Plan and  Management  Stock
Option Plan,  as amended (the "Stock  Option  Plans") and the issuance of Common
Stock  under  the  Company's  Director  Restricted  Stock  Plan  and  Management
Restricted Stock Plan (the "Restricted  Stock Plans",  and collectively with the
Stock Option Plans, the "Plans").

         We assume  that the number and  issuance  of shares of Common  Stock or
options to be granted from time to time  pursuant to the Plans have been or will
be authorized by proper action of the Board of Directors or a Committee  thereof
of the Company or its predecessor and that the number,  issuance and sale of the
Registered Shares to be issued directly or offered from time to time pursuant to
the  exercise  of such  options  will  be  determined  in  accordance  with  the
parameters  described in the Plans, in accordance with the Company's Articles of
Incorporation, as amended (the "Articles"), and applicable Massachusetts law. We
further assume that prior to the issuance of any Registered  Shares,  there will
exist,  under the Articles,  the requisite number of authorized shares of Common
Stock for such issuance  which are unissued and are not  otherwise  reserved for
issuance.






SIS Bancorp, Inc.
September 4, 1996
Page 2

         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
Articles 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.

         This   opinion  is  limited  to  the  laws  of  The   Commonwealth   of
Massachusetts  and we express no  opinion  with  respect to the law of any other
jurisdiction.

         Based on and subject to the foregoing, we are of the opinion that, upon
the issuance by the Company of Registered  Shares either directly or pursuant to
the exercise of options  granted  under either of the Plans and in each instance
upon delivery of certificates  representing the Registered  Shares in the manner
contemplated by the Plans and the authorization relating thereto by the Board or
Committee,  the Registered Shares  represented by such certificates will be duly
authorized, 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  in the  Prospectus
forming a part of the Registration  Statement. 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
Commission promulgated thereunder.

                                Very truly yours,

                                /s/ Sullivan & Worcester LLP

                                SULLIVAN & WORCESTER LLP