June 10, 1998
 
Board of Directors
Plenum Publishing Corporation
233 Spring Street
New York, NY 10013
 
Dear Sirs:
 
    You have requested our opinion as to the fairness, from a financial point of
view, to the holders of common stock, par value $.10 per share ("Common Stock"),
of Plenum Publishing Corporation, a Delaware corporation (the "Company"), of the
consideration to be received by such holders pursuant to the Agreement and Plan
of the Merger dated as of June 10, 1998 (the "Agreement"), among the Company,
Kluwer Boston, Inc., a Massachusetts corporation ("Parent"), and PPC Acquisition
Corp., a Delaware corporation and a wholly owned subsidiary of Parent ("Sub").
In accordance with the Agreement, (i) Parent will cause Sub to make a tender
offer (the "Offer") to purchase all the outstanding shares of Common Stock at a
purchase price of $73.50 per share, net to the seller in cash (the "Cash
Consideration") and (ii) thereafter Sub will be merged with and into the Company
(the "Merger" and, together with the Offer, the "Transaction"), with the Company
continuing as the surviving corporation. In the Merger, shareholders of the
Company will receive the Cash Consideration.
 
    In arriving at our opinion, we have reviewed the Agreement. We have also
reviewed certain publicly available business and financial information relating
to the Company, as well as certain other information, including financial
forecasts, provided to us by the Company. We have discussed the past and current
operations and financial condition and prospects of the Company with its senior
management. We have also considered such other information, financial studies,
analyses, investigations, and financial, economic, market and trading criteria
which we deemed relevant.
 
    We have assumed and relied on the accuracy and completeness of the
information reviewed by us for the purpose of this opinion and we have not
assumed any responsibility for independent verification of such information or
for any independent evaluation or appraisal of the assets of the Company. With
respect to the financial forecasts of the Company, management of the Company has
informed us that such forecasts have been reasonably prepared on bases
reflecting the best currently available estimates and judgments of the Company's
management, and we express no opinion with respect to such forecasts or the
assumptions on which they are based.
 
    Our opinion is necessarily based upon conditions as they exist and can be
evaluated on the date hereof. Our opinion does not address the Company's
underlying business decision to effect the Transaction, and we express no view
on the effect on the Company of the Transaction. Our opinion does not constitute
a recommendation to the Company's holders of Common Stock as to whether they
should tender their shares in the Offer or as to how such holders should vote
with respect to the Merger.
 
    As you are aware, Salomon Brothers Inc and Smith Barney Inc. doing business
as Salomon Smith Barney (collectively with all other entities doing business as
Salomon Smith Barney, "Salomon Smith Barney") are acting as financial advisors
to the Board of Directors of the Company in connection with the Transaction and
will receive a fee for their services, portions of which will be paid following
(i) the execution of the Agreement and (ii) the delivery of this opinion and the
remainder of which is contingent upon consummation of the Offer. In the ordinary
course of business, Salomon Smith Barney and its affiliates may hold or actively
trade the securities of the Company for its own account and for the accounts of
customers and, accordingly, may at any time hold a long or short position in
such securities. In addition, Salomon Smith Barney and its affiliates have
previously rendered certain investment banking and financial advisory services
to the Company for which Salomon Smith Barney has received customary
compensation. Salomon Smith Barney and its current and future affiliates
(including Travelers Group Inc.) may have other business relationships with the
Company and its affiliates.

    Based upon and subject to the foregoing, it is our opinion that, as of the
date of this letter, the Cash Consideration is fair, from a financial point of
view, to the holders of Common Stock.
 
Very truly yours,
 
SALOMON SMITH BARNEY