


                                BAKER & McKENZIE
                                Attorneys at Law

                              One Prudential Plaza
                             130 East Randolph Drive
                             Chicago, Illinois 60601

                                 April 27, 1998


Board of Directors
Transaction Systems Architects, Inc.
224 South 108th Avenue
Omaha, Nebraska 68154

         Re:      Transaction Systems Architects, Inc. (the "Company")

Gentlemen:

     We have acted as your counsel in  connection  with the  registration,  on a
Registration  Statement on Form S-4 (the  "Registration  Statement")  under Rule
462(b) of the  Securities  Act of 1933,  as amended,  of an  additional  224,264
shares of the  Company's  Class A Common  Stock,  $.005 par value per share (the
"Stock"),  to be issued by the Company in connection  with an acquisition by the
Company. We have reviewed the Registration Statement, the charter and by-laws of
the Company,  corporate  proceedings  of the Board of Directors  relating to the
issuance of the shares of Stock, and such other documents, corporate records and
questions of law as we have deemed  necessary  to the  rendering of the opinions
expressed below.
        
     Based upon the foregoing,  we are of the opinion that the 224,264 shares of
Stock to be issued by the Company as described in the Registration Statement are
duly authorized and, when issued and paid for in the manner  contemplated in the
Registration  Statement,  will be legally issued,  fully paid and non-assessable
shares of Class A Common Stock of the Company.

     We hereby  consent to the  reference  to our firm under the caption  "Legal
Matters"  in the  prospectus  incorporated  by  reference  in  the  Registration
Statement and to the filing of this opinion as an exhibit thereto.

                                                              Very truly yours,


                                                              BAKER & McKENZIE


