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                                                                     EXHIBIT 5.2


                          [LETTERHEAD OF BAR & KARRER]


                                                      September 4, 2001

UBS AG,
Bahnhofstrasse 45,
CH-8098 Zurich,
Switzerland.

Dear Sirs,

      We have acted as Swiss counsel to UBS AG, a corporation organized under
the laws of Switzerland (the "Company") in connection with the registration
under the Securities Act of 1933 (the "Act") of $1,390,000,000 principal amount
of debt securities and warrants (collectively, the "Securities").

      In connection with this opinion, we have examined and are familiar with
originals or copies, certified or otherwise identified to our satisfaction, of
such corporate records and other documents, and such certificates or comparable
documents of public officials or officials and representatives of the Company,
as we have considered relevant and necessary as a basis for the opinion
hereinafter set forth.

      Upon the basis of such examination, we advise you that, in our opinion,
all corporate action by the Company related to the Securities, including the
authorization of the issuance of the Securities and of the execution of any
documents in connection therewith, was duly authorized under the Company's
Articles of Association and by-laws as a matter of Swiss law. We further advise
you that, in our opinion:

1. When the Registration Statement relating to the Securities (the "Registration
   Statement") has become effective under the Act, the terms of the debt
   securities and of their issuance and sale have been duly established in
   conformity with the indenture relating to the debt securities so as not to
   violate Swiss law, and the debt securities have been duly executed and
   authenticated in accordance with such indenture and issued and sold as
   contemplated in the Registration Statement, the debt securities will
   constitute valid and legally binding obligations of the Company, subject to
   bankruptcy, insolvency, fraudulent transfer, reorganization, moratorium and
   similar laws of general applicability relating to or affecting creditors'
   rights and to general principles of equity under Swiss law.

2. With respect to the warrants to be issued under the warrant indenture
   referred to in the prospectus forming a part of the Registration Statement,
   when the Registration Statement has become effective under the Act, such
   indenture, substantially in the form filed as an exhibit to the Registration
   Statement, has been duly authorized, executed and delivered, the terms of
   such warrants and of their issuance and sale have been duly established in
   conformity with such indenture so as not to violate Swiss law, and such
   warrants have been duly executed and
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   authenticated in accordance with such indenture and issued and sold as
   contemplated in the Registration Statement, such warrants will constitute
   valid and legally binding obligations of the Company, subject to bankruptcy,
   insolvency, fraudulent transfer, reorganization, moratorium and similar laws
   of general applicability relating to or affecting creditors' rights and to
   general principles of equity under Swiss law.

3. With respect to the warrants to be issued under a warrant agreement as
   described in the prospectus forming a part of the Registration Statement,
   when the Registration Statement has become effective under the Act, the
   warrant agreement under which such warrants are to be issued has been duly
   authorized, executed and delivered in such form as will not violate Swiss
   law, the terms of such warrants and of their issuance and sale have been duly
   established in conformity with such warrant agreement so as not to violate
   Swiss law, and such warrants have been duly executed and authenticated in
   accordance with such warrant agreement and issued and sold as contemplated in
   the Registration Statement, such warrants will constitute valid and legally
   binding obligations of the Company, subject to bankruptcy, insolvency,
   fraudulent transfer, reorganisation, moratorium and similar laws of general
   applicability relating to or affecting creditors' rights and to general
   principles of equity under Swiss law.

      We are members of the Swiss bar and the opinions expressed herein are
limited to the laws of Switzerland. We are expressing no opinion as to the
effect of the laws of any other jurisdiction. With respect to all matters of
United States and New York law, we have relied upon the opinion, dated September
4, 2001 of Sullivan & Cromwell, and our opinion is subject to the same
assumptions, qualifications and limitations with respect to such matters as are
contained in such opinion of Sullivan & Cromwell.

      The opinion is governed by the laws of Switzerland. We hereby consent to
the filing of this opinion as an exhibit to the Registration Statement and to
the references to us under the heading "Validity of the Securities" in the
Prospectus. 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 Act.

      This opinion is furnished by us, as counsel to the Company, in connection
with the filing of the registration of the Securities, and, except as provided
in the immediately preceding paragraph, is not to be used, circulated, quoted or
otherwise referred to for any other purpose without our express written
permission, or relied upon by any other person.

Very truly yours,

Bar & Karrer