CITIGROUP INC
424B2, 1999-03-30
FIRE, MARINE & CASUALTY INSURANCE
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<PAGE>
                                       Filed Pursuant to Rule 424(b)(2)
                                       Registration Statement No. 333-68949
PROSPECTUS SUPPLEMENT
(to prospectus dated March 8, 1999)
 
                                 $1,500,000,000
 
                                [CITIGROUP LOGO]
 
                       $750,000,000 5.80% NOTES DUE 2004
                       $750,000,000 6.20% NOTES DUE 2009
                                ----------------
 
    The 2004 notes offered in this prospectus supplement will mature on March
15, 2004. The 2009 notes offered in this prospectus supplement will mature on
March 15, 2009. The 2004 notes will bear interest at a fixed rate of 5.80% per
annum. The 2009 notes will bear interest at a fixed rate of 6.20% per annum.
Interest on the notes is payable semiannually on March 15 and September 15 of
each year, beginning September 15, 1999. The notes may not be redeemed prior to
maturity, unless changes involving United States taxation occur which could
require Citigroup to pay additional amounts as described under "Description of
Notes."
 
    The notes are being offered globally for sale in the United States, Europe,
Asia and elsewhere where it is lawful to make such offers. Application has been
made to list the notes on the Luxembourg Stock Exchange.
 
    Neither the Securities and Exchange Commission nor any state securities or
insurance commission nor the Luxembourg Stock Exchange has approved or
disapproved of these notes or determined if this prospectus supplement or the
accompanying prospectus is truthful or complete. Any representation to the
contrary is a criminal offense.
                            ------------------------
 
<TABLE>
<CAPTION>
                                                            PUBLIC OFFERING       UNDERWRITING        PROCEEDS TO
                                                                 PRICE              DISCOUNT           CITIGROUP
                                                           ------------------  ------------------  ------------------
<S>                                                        <C>                 <C>                 <C>
Per 2004 Note............................................       99.743%              0.350%             99.393%
                                                              $748,072,500         $2,625,000         $745,447,500
Per 2009 Note............................................       99.698%              0.450%             99.248%
                                                              $747,735,000         $3,375,000         $744,360,000
                                                           ------------------  ------------------  ------------------
Combined Total (before expenses).........................    $1,495,807,500        $6,000,000        $1,489,807,500
</TABLE>
 
    Interest on the notes will accrue from March 31, 1999 to date of delivery.
                            ------------------------
 
    The underwriters are offering the notes subject to various conditions. The
underwriters expect that the notes will be ready for delivery in book-entry form
only through The Depository Trust Company, Cedelbank or the Euroclear System on
or about March 31, 1999.
 
    The notes are not deposits or savings accounts but are unsecured debt
obligations of Citigroup and are not insured by the Federal Deposit Insurance
Corporation or any other governmental agency or instrumentality.
                            ------------------------
 
              THE UNDERWRITERS OFFERING THE 5.80% NOTES DUE 2004:
                              SALOMON SMITH BARNEY
 
<TABLE>
<S>                              <C>                              <C>
Credit Suisse First Boston                                                  Chase Securities Inc.
Donaldson, Lufkin & Jenrette                                                      Lehman Brothers
                                               SG
ABN AMRO Incorporated                                                            Barclays Capital
BBV Securities, Inc.                                               Commerzbank Aktiengesellschaft
First Chicago Capital Markets,                                        First Union Capital Markets
Inc.                                                                                        Corp.
      A BANK ONE Company             Rabobank International
</TABLE>
 
              THE UNDERWRITERS OFFERING THE 6.20% NOTES DUE 2009:
                              SALOMON SMITH BARNEY
 
<TABLE>
<S>                                              <C>
ABN AMRO Incorporated                                                   Bear, Stearns & Co. Inc.
Goldman, Sachs & Co.                                                         Merrill Lynch & Co.
</TABLE>
 
                     NationsBanc Montgomery Securities LLC
 
March 26, 1999
<PAGE>
                               TABLE OF CONTENTS
 
                             PROSPECTUS SUPPLEMENT
 
<TABLE>
<CAPTION>
                                                                                                               PAGE
                                                                                                             ---------
<S>                                                                                                          <C>
The Company................................................................................................        S-4
Selected Historical Financial Data.........................................................................        S-4
Capitalization.............................................................................................        S-5
Use of Proceeds............................................................................................        S-6
Description of Notes.......................................................................................        S-6
United States Tax Documentation Requirements...............................................................       S-15
United States Federal Income Tax Considerations for Non-United States Holders..............................       S-17
Underwriting...............................................................................................       S-19
Notice to Canadian Residents...............................................................................       S-21
Directors and Executive Officers of Citigroup Inc..........................................................       S-22
Legal Opinions.............................................................................................       S-23
General Information........................................................................................       S-23
 
                                                      PROSPECTUS
Prospectus Summary.........................................................................................          2
Citigroup Inc..............................................................................................          7
Use of Proceeds and Hedging................................................................................          8
Ratio of Income to Fixed Charges and Ratio of Income to Combined Fixed Charges Including Preferred Stock
  Dividends................................................................................................          9
European Monetary Union....................................................................................         10
Description of Debt Securities.............................................................................         11
Description of Index Warrants..............................................................................         19
Description of Capital Stock...............................................................................         22
Description of Preferred Stock.............................................................................         25
Description of Depositary Shares...........................................................................         28
Book-Entry Procedures and Settlement.......................................................................         31
Plan of Distribution.......................................................................................         33
ERISA Matters..............................................................................................         35
Legal Matters..............................................................................................         35
Experts....................................................................................................         35
</TABLE>
 
                            ------------------------
 
    You should rely only on the information contained or incorporated by
reference in this prospectus supplement and the accompanying prospectus.
Citigroup has not authorized any other person to provide you with different
information. If anyone provides you with different or inconsistent information,
you should not rely on it. Citigroup is not making an offer to sell the notes in
any jurisdiction where their offer and sale is not permitted. You should assume
that the information appearing in this prospectus supplement and the
accompanying prospectus, as well as information Citigroup previously filed with
the Securities and Exchange Commission and incorporated by reference, is
accurate only as of the date of the applicable document.
 
                            ------------------------
 
    The notes are offered globally for sale in those jurisdictions in the United
States, Europe, Asia and elsewhere where it is lawful to make such offers.
 
                                      S-2
<PAGE>
    This prospectus supplement and the accompanying prospectus include
information provided in order to comply with the rules governing the listing of
securities on the Luxembourg Stock Exchange. Citigroup is responsible for the
accuracy of the information contained or incorporated by reference in this
prospectus supplement and the accompanying prospectus. Citigroup confirms, after
reasonable inquiry, that to the best of its knowledge and belief it has not
omitted any other fact that would make any statement contained or incorporated
by reference in this prospectus supplement misleading in any material respect.
 
    THE LUXEMBOURG STOCK EXCHANGE TAKES NO RESPONSIBILITY FOR THE CONTENTS OF
THIS DOCUMENT, MAKES NO REPRESENTATION AS TO ITS ACCURACY OR COMPLETENESS AND
EXPRESSLY DISCLAIMS ANY LIABILITY WHATSOEVER FOR ANY LOSS HOWSOEVER ARISING FROM
OR IN RELIANCE UPON THE WHOLE OR ANY PART OF THE CONTENTS OF THIS PROSPECTUS
SUPPLEMENT AND THE ACCOMPANYING PROSPECTUS.
 
    The distribution of this prospectus supplement and the accompanying
prospectus and the offering of the notes in certain jurisdictions may be
restricted by law. If you possess this prospectus supplement and the
accompanying prospectus, you should find out about and observe any of these
restrictions. This prospectus supplement and the accompanying prospectus are not
an offer to sell these securities and are not soliciting an offer to buy these
securities in any jurisdiction where the offer or sale is not permitted or where
the person making the offer or sale is not qualified to do so or to any person
to whom it is not permitted to make such offer or sale. See "Underwriting."
 
    References in this prospectus supplement to "dollars," "$" and "U.S.$" are
to United States dollars.
 
                                      S-3
<PAGE>
                                  THE COMPANY
 
    Citigroup Inc. is a diversified holding company whose businesses provide a
broad range of financial services to consumer and corporate customers around the
world. Citigroup's activities are conducted through Global Consumer, Global
Corporate and Investment Bank, Asset Management, and Investment Activities.
 
    Citigroup was incorporated in Delaware in 1988 under the name Commercial
Credit Group, Inc. On October 8, 1998, Citigroup changed its name from Travelers
Group Inc. to Citigroup Inc. in connection with the merger of Citicorp into a
subsidiary of Citigroup.
 
    The principal office of Citigroup is located at 153 East 53rd Street, New
York, NY 10043, and its telephone number is (212) 559-1000.
 
                       SELECTED HISTORICAL FINANCIAL DATA
 
    We are providing or incorporating by reference in this prospectus supplement
selected historical financial information of Citigroup. All periods have been
restated where appropriate to reflect the merger of Citicorp into a subsidiary
of Travelers Group Inc. on October 8, 1998 and the merger of Salomon Inc with a
subsidiary of Travelers Group Inc. on November 28, 1997, each of which was
accounted for as a pooling of interests. The results of the property casualty
business acquired from subsidiaries of Aetna Service, Inc. are included from the
date of acquisition, April 2, 1996. We derived this information from the
consolidated financial statements of Citigroup, after giving effect to the
Citicorp and Salomon Inc mergers, for each of the periods presented. The
information is only a summary and should be read together with the financial
information incorporated by reference in this prospectus supplement. See "Where
You Can Find More Information" on page 6 of the accompanying prospectus.
 
<TABLE>
<CAPTION>
                                                                 AT OR FOR THE YEAR ENDED DECEMBER 31,
                                                       ----------------------------------------------------------
                                                          1998        1997        1996        1995        1994
                                                       ----------  ----------  ----------  ----------  ----------
                                                            (DOLLARS IN MILLIONS, EXCEPT PER SHARE AMOUNTS)
<S>                                                    <C>         <C>         <C>         <C>         <C>
INCOME STATEMENT DATA:
  Total revenues.....................................  $   76,431  $   72,306  $   65,101  $   58,957  $   54,369
  Total revenues, net of interest expense............      48,936      47,782      43,765      36,567      31,841
  Income from continuing operations(1)...............       5,807       6,705       7,073       5,610       4,182
  Net income.........................................       5,807       6,705       6,739       5,760       4,362
  Dividends declared per common share(2).............       0.555       0.400       0.300       0.267       0.192
 
BALANCE SHEET DATA:
  Total assets.......................................  $  668,641  $  697,384  $  626,906  $  559,146  $  537,540
  Total deposits.....................................     228,649     199,121     184,955     167,131     155,726
  Long-term debt.....................................      48,671      47,387      43,246      40,723      40,171
  Total stockholders' equity.........................      42,708      41,851      38,416      35,183      29,945
</TABLE>
 
- ------------------------------
 
(1) The years ended December 31, 1998 and 1997 include net restructuring charges
    (and in 1998 merger-related costs) of $795 million ($535 million after-tax)
    and $1,718 million ($1,046 million after-tax), respectively.
 
(2) Amounts represent Travelers Group's historical dividends per common share.
 
                                      S-4
<PAGE>
                                 CAPITALIZATION
 
    The following table sets forth the consolidated capitalization of Citigroup
at December 31, 1998, and as adjusted to give effect to the issuance and sale of
the notes, and no other change in the consolidated capitalization of Citigroup
since December 31, 1998 is reflected therein. Since December 31, 1998, there has
not been any material adverse change in the information set forth below, except
as described elsewhere in this prospectus supplement, in the accompanying
prospectus or in any of the documents incorporated by reference in this
prospectus supplement. You should read this table together with Citigroup's
historical audited consolidated financial statements, including the notes to
such financial statements, which are incorporated by reference in this
prospectus supplement and the accompanying prospectus.
 
<TABLE>
<CAPTION>
                                                                                           AT DECEMBER 31, 1998
                                                                                         ------------------------
                                                                                         OUTSTANDING  AS ADJUSTED
                                                                                         -----------  -----------
                                                                                          (DOLLARS IN MILLIONS)
<S>                                                                                      <C>          <C>
Debt:
  Investment banking and brokerage borrowings..........................................   $  14,040    $  14,040
  Short-term borrowings................................................................      16,112       16,112
  Long-term debt.......................................................................      48,671       50,171
                                                                                         -----------  -----------
    Total debt.........................................................................      78,823       80,323
                                                                                         -----------  -----------
Redeemable Preferred Stock--Series I...................................................         140          140
                                                                                         -----------  -----------
Company or subsidiary obligated mandatorily redeemable securities of subsidiary trusts
 holding solely junior subordinated debt securities of--
  Company..............................................................................       1,700        1,700
  Subsidiaries.........................................................................       2,620        2,620
Stockholders' equity:
  Preferred stock at aggregate liquidation value.......................................       2,313        2,313
  Common stock and additional paid-in capital (net of treasury stock)..................       4,140        4,140
  Retained earnings....................................................................      35,971       35,971
  Accumulated other changes in equity from nonowner sources............................         781          781
  Unearned compensation................................................................        (497)        (497)
                                                                                         -----------  -----------
    Total stockholders' equity.........................................................      42,708       42,708
                                                                                         -----------  -----------
  Total capitalization.................................................................   $ 125,991    $ 127,491
                                                                                         -----------  -----------
                                                                                         -----------  -----------
</TABLE>
 
                                      S-5
<PAGE>
                                USE OF PROCEEDS
 
    Citigroup will use the net proceeds it receives from the sale of the notes
for general corporate purposes, which may include (1) capital contributions to
subsidiaries of Citigroup and/or (2) the reduction or refinancing of borrowings
of Citigroup or its subsidiaries. Citigroup expects to incur additional
indebtedness in the future.
 
                              DESCRIPTION OF NOTES
 
    THE FOLLOWING DESCRIPTION OF THE PARTICULAR TERMS OF THE NOTES SUPPLEMENTS
THE DESCRIPTION OF THE GENERAL TERMS SET FORTH IN THE ACCOMPANYING PROSPECTUS.
IT IS IMPORTANT FOR YOU TO CONSIDER THE INFORMATION CONTAINED IN THE
ACCOMPANYING PROSPECTUS AND THIS PROSPECTUS SUPPLEMENT BEFORE MAKING YOUR
DECISION TO INVEST IN THE NOTES. IF ANY SPECIFIC INFORMATION REGARDING THE NOTES
IN THIS PROSPECTUS SUPPLEMENT IS INCONSISTENT WITH THE MORE GENERAL TERMS OF THE
NOTES DESCRIBED IN THE PROSPECTUS, YOU SHOULD RELY ON THE INFORMATION CONTAINED
IN THIS PROSPECTUS SUPPLEMENT.
 
GENERAL
 
    Each of the 2004 notes and the 2009 notes offered pursuant to this
prospectus supplement is a series of senior debt securities issued under
Citigroup's senior debt indenture. The 2004 notes will be limited to an
aggregate principal amount of $750,000,000, and the 2009 notes will be limited
to an aggregate principal amount of $750,000,000. The notes will be issued only
in fully registered form without coupons, in denominations of $1,000 and whole
multiples of $1,000. All the notes are unsecured obligations of Citigroup and
will rank equally with all other unsecured senior indebtedness of Citigroup,
whether currently existing or hereinafter created.
 
    As of the date of this prospectus supplement, Citigroup may offer an
aggregate principal amount of $3,900,000,000 of additional debt securities under
the registration statement of which this prospectus supplement and the
accompanying prospectus form a part. Citigroup may, without notice to or consent
of the holders or beneficial owners of the notes, issue additional notes having
the same ranking, interest rate, maturity and other terms as either the 2004
notes or 2009 notes. Any such additional notes issued could be considered part
of the same series of notes under the indenture as the 2004 notes or 2009 notes,
as the case may be.
 
    The notes are not redeemable prior to maturity, except upon the occurrence
of the tax events described below. See "--Redemption for Tax Purposes."
 
    The 2004 notes will bear interest at a fixed rate per year of 5.80%,
starting on March 31, 1999 and ending on their maturity date, which is March 15,
2004. The 2009 notes will bear interest at a fixed rate per year of 6.20%,
starting on March 31, 1999 and ending on their maturity date, which is March 15,
2009. Interest on the notes will be payable semiannually on March 15 and
September 15 of each year, starting on September 15, 1999. All payments of
interest will be made to the persons in whose names the notes are registered on
the March 1 or September 1 preceding the interest payment date.
 
    Interest on the notes will be computed on the basis of a 360 day year
comprised of twelve 30 day months. All dollar amounts resulting from this
calculation will be rounded to the nearest cent.
 
    Payments of principal and interest on notes issued in book-entry form will
be made as described below under "--Book-Entry Notes." Payments of principal and
interest on notes issued in definitive form, if any, will be made as described
below under "--Definitive Notes and Transfers."
 
    The notes are subject to the defeasance provisions explained in the
accompanying prospectus under "Description of Debt Securities--Defeasance."
 
                                      S-6
<PAGE>
BOOK-ENTRY NOTES
 
    BOOK-ENTRY NOTES; THE DEPOSITORY TRUST COMPANY
 
    Except under the limited circumstances described below, all notes will be
book-entry notes. This means that the actual purchasers of the notes will not be
entitled to have the notes registered in their names and will not be entitled to
receive physical delivery of the notes in definitive (paper) form. Instead, upon
issuance, all the notes will be represented by one or more fully registered
global notes.
 
    Each global note will be deposited with The Depository Trust Company, a
securities depositary, and will be registered in the name of DTC's nominee, Cede
& Co. No global note representing book-entry notes may be transferred except as
a whole by DTC to a nominee of DTC, or by a nominee of DTC to another nominee of
DTC. Thus, DTC will be the only registered holder of the notes and will be
considered the sole representative of the beneficial owners of the notes for
purposes of the indenture.
 
    The registration of the global notes in the name of Cede & Co. will not
affect beneficial ownership and is performed merely to facilitate subsequent
transfers. The book-entry system, which is also the system through which most
publicly traded common stock is held in the United States, is used because it
eliminates the need for physical movement of securities certificates. The laws
of some jurisdictions, however, may require some purchasers to take physical
delivery of their notes in definitive form. These laws may impair the ability of
holders to transfer book-entry notes.
 
    Purchasers of notes in the United States may hold interests in the global
notes only through DTC, if they are participants in such system. Purchasers may
also hold interests indirectly through securities intermediaries--such as banks,
brokerage houses and other institutions that maintain securities accounts for
customers--that have accounts with DTC or its nominee ("participants").
Purchasers of notes in Europe can hold interests in the global notes only
through Cedelbank, SOCIETE ANONYME, or through Morgan Guaranty Trust Company of
New York, Brussels Office, as operator of the Euroclear System, if they are
participants in these systems or indirectly through organizations that are
participants in these systems.
 
    Because DTC will be the only registered owner of the global notes, Cedelbank
and Euroclear will hold positions through their respective U.S. depositaries,
which in turn will hold positions on the books of DTC. Citibank, N.A. will act
as U.S. depositary for Cedelbank, and The Chase Manhattan Bank will act as U.S.
depositary for Euroclear. For information on how accounts of ownership of notes
held through DTC are recorded, please refer to "Book-Entry Procedures and
Settlement" beginning on page 31 of the accompanying prospectus.
 
    Citigroup, the trustee and all of their agents will not be liable for the
accuracy of, or responsible for maintaining, supervising or reviewing, DTC's
records or any participant's records relating to book-entry notes. Citigroup,
the trustee and all of their agents also will not be responsible or liable for
payments made on account of the book-entry notes.
 
    In this prospectus supplement, unless and until definitive (paper) notes are
issued to the beneficial owners as described below, all references to "holders"
of notes shall mean DTC. Citigroup, the trustee and any paying agent, transfer
agent or registrar may treat DTC as the absolute owner of the notes for all
purposes.
 
    Citigroup will make all distributions of principal and interest on their
notes to DTC. Citigroup will send all required reports and notices solely to DTC
as long as DTC is the registered holder of the notes. DTC and its participants
are generally required by law to receive and transmit all distributions, notices
and directions from Citigroup and the trustee to the beneficial owners through a
chain of intermediaries. Purchasers of the notes will not receive written
confirmation from DTC of their purchases. However, beneficial owners of
book-entry notes are expected to receive written
 
                                      S-7
<PAGE>
confirmations providing details of the transaction, as well as periodic
statements of their holdings, from the participants or indirect participants
through which they entered into the transaction.
 
    Similarly, Citigroup and the trustee will accept notices and directions
solely from DTC. Therefore, in order to exercise any rights of a holder of notes
under the indenture, each person owning a beneficial interest in the notes must
rely on the procedures of DTC and, in some cases, Cedelbank or Euroclear. If the
beneficial owner is not a participant in the applicable system, then it must
rely on the procedures of the participant through which that person owns its
interest. DTC has advised Citigroup that it will take actions under the
indenture only at the direction of its participants, which in turn will act only
at the direction of the beneficial owners. Some of these actions, however, may
conflict with actions DTC takes at the direction of other participants and
beneficial owners.
 
    Notices and other communications by DTC to participants, by participants to
indirect participants, and by participants and indirect participants to
beneficial owners will be governed by arrangements among them.
 
    Book-entry notes may be more difficult to pledge because of the lack of a
physical note. Beneficial owners may experience delays in receiving
distributions on their notes since distributions will initially be made to DTC
and must then be transferred through the chain of intermediaries to the
beneficial owner's account.
 
    For background information on DTC, please refer to "Book-Entry Procedures
and Settlement" beginning on page 31 of the accompanying prospectus.
 
    CEDELBANK
 
    Cedelbank is registered as a bank in Luxembourg and is subject to regulation
by the Banque Centrale du Luxembourg, the Luxembourg Central Bank, which
supervises Luxembourg banks. Cedelbank holds securities for its customers and
facilitates the clearance and settlement of securities transactions by
electronic book-entry transfers between their accounts. Cedelbank provides
various services, including safekeeping, administration, clearance and
settlement of internationally traded securities and securities lending and
borrowing. Cedelbank also deals with domestic securities markets in over 30
countries through established depository and custodial relationships. Cedelbank
has established an electronic bridge with Euroclear in Brussels to facilitate
settlement of trades between Cedelbank and Euroclear. Cedelbank currently
accepts over 110,000 securities issues on its books.
 
    Cedelbank's customers are worldwide financial institutions including
underwriters, securities brokers and dealers, banks, trust companies and
clearing corporations. Cedelbank's U.S. customers are limited to securities
brokers and dealers, and banks. Currently, Cedelbank has approximately 2,000
customers located in over 80 countries, including all major European countries,
Canada and the United States. Indirect access to Cedelbank is available to other
institutions that clear through or maintain a custodial relationship with an
account holder of Cedelbank.
 
    EUROCLEAR
 
    Euroclear was created in 1968 to hold securities for participants of
Euroclear and to clear and settle transactions between Euroclear participants
through simultaneous electronic book-entry delivery against payment. This system
eliminates the need for physical movement of notes and any risk from lack of
simultaneous transfers of securities and cash. Euroclear includes various other
services, including securities lending and borrowing and interfaces with
domestic markets in several countries. The Euroclear Operator is the Brussels,
Belgium office of Morgan Guaranty Trust Company of New York, under a contract
with Euro-clear Clearance Systems S.C., a Belgian cooperative corporation (the
"Cooperative"). The Euroclear Operator conducts all operations. All Euroclear
securities clearance accounts and Euroclear cash accounts are accounts with the
Euroclear Operator, not the Cooperative.
 
                                      S-8
<PAGE>
The Cooperative establishes policy for Euroclear on behalf of Euroclear
participants. Euroclear participants include banks, including central banks,
securities brokers and dealers and other professional financial intermediaries
and may include the underwriter. Indirect access to Euroclear is also available
to other firms that clear through or maintain a custodial relationship with a
Euroclear participant, either directly or indirectly.
 
    The Euroclear Operator is the Belgian branch of a New York banking
corporation which is a member bank of the Federal Reserve System. As such, it is
regulated and examined by the Board of Governors of the Federal Reserve System
and the New York State Banking Department, as well as the Belgian Banking
Commission.
 
    Securities clearance accounts and cash accounts with the Euroclear Operator
are governed by the Terms and Conditions Governing Use of Euroclear and the
related Operating Procedures of the Euroclear System, and applicable Belgian
law. These Terms and Conditions govern transfers of securities and cash within
Euroclear, withdrawals of securities and cash from Euroclear, and receipts of
payments with respect to securities in Euroclear. All securities in Euroclear
are held on a fungible basis without attribution of specific notes to specific
securities clearance accounts. The Euroclear Operator acts under the Terms and
Conditions only on behalf of Euroclear participants, and has no record of or
relationship with persons holding through Euroclear participants.
 
    The foregoing information about DTC, Cedelbank and Euroclear has been
provided by each of them for informational purposes only and is not intended to
serve as a representation, warranty, or contract modification of any kind.
 
    GLOBAL CLEARANCE AND SETTLEMENT PROCEDURES
 
    Initial settlement for the notes will be made in immediately available
funds. Secondary market trading between DTC participants will occur in the
ordinary way, in accordance with DTC's rules, and will be settled in immediately
available funds using DTC's same-day funds settlement system. Secondary market
trading between Cedelbank participants and/or Euroclear participants will occur
in the ordinary way, in accordance with the applicable rules and operating
procedures of Cedelbank and Euroclear, and will be settled using the procedures
applicable to conventional eurobonds in immediately available funds.
 
    Cross-market transfers between persons holding directly or indirectly
through DTC, on the one hand, and directly or indirectly through Cedelbank or
Euroclear participants, on the other, will be effected in DTC, in accordance
with DTC's rules, on behalf of the relevant European international clearing
system by the U.S. depositaries. However, such cross-market transactions will
require delivery of instructions to the relevant European international clearing
system by the counterparty in this system in accordance with its rules and
procedures and within its established deadlines, European time. The relevant
European international clearing system will, if the transaction meets its
settlement requirements, deliver instructions to its U.S. depositary to take
action to effect final settlement on its behalf by delivering or receiving notes
in DTC, and making or receiving payment in accordance with normal procedures for
same-day funds settlement applicable to DTC. Cedelbank participants and
Euroclear participants may not deliver instructions directly to DTC.
 
    Because of time-zone differences, credits of notes received in Cedelbank or
Euroclear as a result of a transaction with a DTC participant will be made
during subsequent securities settlement processing and will be credited the
business day following the DTC settlement date. These credits or any
transactions in such notes settled during such processing will be reported to
the relevant Euroclear or Cedelbank participants on that business day. Cash
received in Cedelbank or Euroclear as a result of sales of notes by or through a
Cedelbank participant or a Euroclear participant to a DTC participant will be
received with value on the DTC settlement date, but will be available in the
relevant Cedelbank or Euroclear cash account only as of the business day
following settlement in DTC.
 
                                      S-9
<PAGE>
    Although DTC, Cedelbank and Euroclear have agreed to the foregoing
procedures in order to facilitate transfers of notes among participants of DTC,
Cedelbank and Euroclear, they are under no obligation to perform or continue to
perform these procedures, and these procedures may be discontinued at any time.
 
    DISTRIBUTIONS ON BOOK-ENTRY NOTES
 
    Citigroup will make all distributions of principal and interest on
book-entry notes to DTC. Upon receipt of any payment of principal or interest,
DTC will immediately credit the accounts of its participants on its book-entry
registration and transfer system. DTC will credit those accounts in proportion
to the participants' respective beneficial interests in the principal amount of
the global note as shown on the records of DTC. Payments by participants to
beneficial owners of book-entry notes will be governed by standing instructions
and customary practices, as is now the case with securities held for the
accounts of customers in bearer form or registered in "street name," and will be
the responsibility of those participants.
 
    Distributions on the book-entry notes held beneficially through Cedelbank
will be credited to Cedelbank participants, in accordance with Cedelbank's rules
and procedures, to the extent received by its U.S. depositary.
 
    Distributions on book-entry notes held beneficially through Euroclear will
be credited to Euroclear participants, in accordance with the Terms and
Conditions, to the extent received by its U.S. depositary.
 
DEFINITIVE NOTES AND PAYING AGENTS
 
    If any of the events described under "Book-Entry Procedures and Settlement"
on page 31 of the accompanying prospectus occurs, or if both Cedelbank and
Euroclear notify Citigroup that they are unwilling or unable to continue as a
clearing system in connection with the notes, then the beneficial owners will be
notified through the chain of intermediaries that definitive notes are
available. Beneficial owners of book-entry notes will then be entitled (1) to
receive physical delivery in certificated form of definitive notes equal in
principal amount to their beneficial interest and (2) to have the definitive
notes registered in their names. The definitive notes will be issued in
denominations of $1,000 and whole multiples of $1,000 in excess of that amount.
Definitive notes will be registered in the name or names of the person or
persons DTC specifies in a written instruction to the registrar of the notes.
DTC may base its written instruction upon directions it receives from its
participants. Thereafter, the holders of the definitive notes will be recognized
as the "holders" of the notes under the indenture.
 
    The indenture provides for the replacement of a mutilated, lost, stolen or
destroyed definitive note, so long as the applicant furnishes to Citigroup and
the trustee such security or indemnity and such evidence of ownership as they
may require.
 
    In the event definitive notes are issued, the holders of definitive notes
will be able to receive payments of principal and interest on their notes at the
office of Citigroup's paying agent maintained in the Borough of Manhattan, and,
if the definitive notes are listed on the Luxembourg Stock Exchange, at the
offices of the paying agent in Luxembourg. Citigroup also has the option of
mailing checks to the registered holders of the notes.
 
    Citigroup's paying agent in the Borough of Manhattan is currently the
corporate trust office of The Bank of New York, located at 101 Barclay Street,
Floor 21 West, New York, New York. Citigroup's paying agent and transfer agent
in Luxembourg is Banque Internationale a Luxembourg S.A., currently located at
69 route d'Esch, L-1470 Luxembourg. As long as the notes are listed on the
Luxembourg Stock Exchange, Citigroup will maintain a paying agent and transfer
agent in Luxembourg. Any change in the Luxembourg paying agent and transfer
agent will be published in London and Luxembourg. See "--Notices."
 
                                      S-10
<PAGE>
    In the event definitive notes are issued, the holders of definitive notes
will be able to transfer their notes, in whole or in part, by surrending the
notes for registration of transfer at the office of The Bank of New York and, so
long as definitive notes are listed on the Luxembourg Stock Exchange, at the
offices of the paying agent in Luxembourg, duly endorsed by or accompanied by a
written instrument of transfer in form satisfactory to Citigroup and the
securities registrar. Upon surrender, Citigroup will execute, and the trustee
will authenticate and deliver new notes to the designated transferee in the
amount being transferred, and a new note for any amount not being transferred
will be issued to the transferor. Citigroup will not charge any fee for the
registration of transfer or exchange, except that Citigroup may require the
payment of a sum sufficient to cover any applicable tax or other governmental
charge payable in connection with the transfer.
 
NOTICES
 
    Notices to holders of the notes will be made by first class mail, postage
prepaid, to the addresses that appear on the register of Citigroup. So long as
the notes are listed on the Luxembourg Stock Exchange, notices to non-United
States residents will be made by publication in authorized newspapers in London
and in Luxembourg. It is expected that publication will be made in London in the
FINANCIAL TIMES and in Luxembourg in the LUXEMBURGER WORT. Any notice will be
deemed to have been given on the date of publication or, if published more than
once, on the date of the first publication.
 
PAYMENT OF ADDITIONAL AMOUNTS
 
    OBLIGATION TO PAY ADDITIONAL AMOUNTS
 
    Citigroup will pay additional amounts to the beneficial owner of any note
that is a non-United States person in order to ensure that every net payment on
such note will not be less than the amount then due and payable. For this
purpose, a "net payment" on a note means a payment by Citigroup or a paying
agent, including payment of principal and interest, after deduction for any
present or future tax, assessment or other governmental charge of the United
States. These additional amounts will constitute additional interest on the
note.
 
    EXCEPTIONS
 
    Citigroup will not be required to pay additional amounts, however, in any of
the circumstances described in items (1) through (12) below.
 
    (1) Additional amounts will not be payable if a payment on a note is reduced
       as a result of any tax, assessment or other governmental charge that is
       imposed or withheld solely by reason of the beneficial owner:
 
       -  having a relationship with the United States as a citizen, resident or
          otherwise;
 
       -  having had such a relationship in the past; or
 
       -  being considered as having or having had such a relationship.
 
    (2) Additional amounts will not be payable if a payment on a note is reduced
       as a result of any tax, assessment or other governmental charge that is
       imposed or withheld solely by reason of the beneficial owner:
 
       -  being treated as present in or engaged in a trade or business in the
          United States;
 
       -  being treated as having been present in or engaged in a trade or
          business in the United Sates in the past; or
 
       -  having or having had a permanent establishment in the United States.
 
                                      S-11
<PAGE>
    (3) Additional amounts will not be payable if a payment on a note is reduced
       as a result of any tax, assessment or other governmental charge that is
       imposed or withheld solely by reason of the beneficial owner being or
       having been a:
 
       -  personal holding company;
 
       -  foreign personal holding company;
 
       -  foreign private foundation or other foreign tax-exempt organization;
 
       -  passive foreign investment company;
 
       -  controlled foreign corporation; or
 
       -  corporation which has accumulated earnings to avoid United States
          federal income tax.
 
    (4) Additional amounts will not be payable if a payment on a note is reduced
       as a result of any tax, assessment or other governmental charge that is
       imposed or withheld solely by reason of the beneficial owner owning or
       having owned, actually or constructively, 10 percent or more of the total
       combined voting power of all classes of stock of Citigroup entitled to
       vote.
 
For purposes of items (1) through (4) above, "beneficial owner" means a
fiduciary, settlor, beneficiary, member or shareholder of the holder if the
holder is an estate, trust, partnership, limited liability company, corporation
or other entity, or a person holding a power over an estate or trust
administered by a fiduciary holder.
 
    (5) Additional amounts will not be payable to any beneficial owner of a note
       that is a:
 
       -  fiduciary;
 
       -  partnership;
 
       -  limited liability company; or
 
       -  other fiscally transparent entity
 
       or that is not the sole beneficial owner of the note, or any portion of
       the note. However, this exception to the obligation to pay additional
       amounts will only apply to the extent that a beneficiary or settlor in
       relation to the fiduciary, or a beneficial owner or member of the
       partnership, limited liability company or other fiscally transparent
       entity, would not have been entitled to the payment of an additional
       amount had the beneficiary, settlor, beneficial owner or member received
       directly its beneficial or distributive share of the payment.
 
    (6) Additional amounts will not be payable if a payment on a note is reduced
       as a result of any tax, assessment or other governmental charge that is
       imposed or withheld solely by reason of the failure of the beneficial
       owner or any other person to comply with applicable certification,
       identification, documentation or other information reporting
       requirements. This exception to the obligation to pay additional amounts
       will only apply if compliance with such reporting requirements is
       required by statute or regulation of the United States or by an
       applicable income tax treaty to which the United States is a party as a
       precondition to exemption from such tax, assessment or other governmental
       charge.
 
    (7) Additional amounts will not be payable if a payment on a note is reduced
       as a result of any tax, assessment or other governmental charge that is
       collected or imposed by any method other than by withholding from a
       payment on a note by Citigroup or a paying agent.
 
    (8) Additional amounts will not be payable if a payment on a note is reduced
       as a result of any tax, assessment or other governmental charge that is
       imposed or withheld by reason of a change in law, regulation, or
       administrative or judicial interpretation that becomes effective
 
                                      S-12
<PAGE>
       more than 15 days after the payment becomes due or is duly provided for,
       whichever occurs later.
 
    (9) Additional amounts will not be payable if a payment on a note is reduced
       as a result of any tax, assessment or other governmental charge that is
       imposed or withheld by reason of the presentation by the beneficial owner
       of a note for payment more than 30 days after the date on which such
       payment becomes due or is duly provided for, whichever occurs later.
 
    (10) Additional amounts will not be payable if a payment on a note is
       reduced as a result of any:
 
       -  estate tax;
 
       -  inheritance tax;
 
       -  gift tax;
 
       -  sales tax;
 
       -  excise tax;
 
       -  transfer tax;
 
       -  wealth tax;
 
       -  personal property tax; or
 
       -  any similar tax, assessment or other governmental charge.
 
    (11) Additional amounts will not be payable if a payment on a note is
       reduced as a result of any tax, assessment or other governmental charge
       required to be withheld by any paying agent from a payment of principal
       or interest on a note if such payment can be made without such
       withholding by any other paying agent.
 
    (12) Additional amounts will not be payable if a payment on a note is
       reduced as a result of any combination of items (1) through (11) above.
 
    Except as specifically provided under "Payment of Additional Amounts" and
under "Redemption for Tax Purposes" below, Citigroup will not be required to
make any payment of any tax, assessment or other governmental charge imposed by
any government or a political subdivision or taxing authority of such
government.
 
    RELEVANT DEFINITIONS
 
    As used in this prospectus supplement, "United States person" means:
 
    -  any individual who is a citizen or resident of the United States;
 
    -  any corporation, partnership or other entity created or organized in or
       under the laws of the United States;
 
    -  any estate if the income of such estate falls within the federal income
       tax jurisdiction of the United States regardless of the source of such
       income; and
 
    -  any trust if a United States court is able to exercise primary
       supervision over its administration and one or more United States persons
       have the authority to control all of the substantial decisions of the
       trust.
 
    Additionally, "non-United States person" means a person who is not a United
States person, and "United States" means the United States of America, including
the States and the District of Columbia, its territories, its possessions and
other areas within its jurisdiction.
 
                                      S-13
<PAGE>
REDEMPTION FOR TAX PURPOSES
 
    REDEMPTION PROCEDURE
 
    Citigroup may, at its option, redeem the notes as a whole, but not in part,
on not less than 30 nor more than 60 days' prior notice, only in the
circumstances described in items (1) or (2) below under "Redemption
Circumstances." To redeem, Citigroup must pay a redemption price equal to 100%
of the principal amount of the notes, together with accrued interest to the
redemption date.
 
    REDEMPTION CIRCUMSTANCES
 
    There are two sets of circumstances in which Citigroup may redeem the notes
in the manner described above under "Redemption Procedure:"
 
(1) Citigroup may redeem the notes if:
 
    -  Citigroup becomes or will become obligated to pay additional amounts as
       described under "Payment of Additional Amounts" above;
 
    -  the obligation to pay additional amounts arises as a result of any change
       in the laws, regulations or rulings of the United States, or an official
       position regarding the application or interpretation of such laws,
       regulations or rulings, which change is announced or becomes effective on
       or after the date of this prospectus supplement; and
 
    -  Citigroup determines, in its business judgment, that the obligation to
       pay such additional amounts cannot be avoided by the use of reasonable
       measures available to it, other than substituting the obligor under the
       notes or taking any action that would entail a material cost to
       Citigroup.
 
(2) Citigroup may also redeem the notes if:
 
    -  any act is taken by a taxing authority of the United States on or after
       the date of this prospectus supplement, whether or not such act is taken
       in relation to Citigroup or any affiliate, that results in a substantial
       probability that Citigroup will or may be required to pay additional
       amounts as described under "Payment of Additional Amounts" above;
 
    -  Citigroup determines, in its business judgment, that the obligation to
       pay such additional amounts cannot be avoided by the use of reasonable
       measures available to it, other than substituting the obligor under the
       notes or taking any action that would entail a material cost to
       Citigroup; and
 
    -  Citigroup receives an opinion of independent counsel to the effect that
       an act taken by a taxing authority of the United States results in a
       substantial probability that Citigroup will or may be required to pay the
       additional amounts described under "Payment of Additional Amounts" above,
       and delivers to the trustee a certificate, signed by a duly authorized
       officer, stating that based on such opinion Citigroup is entitled to
       redeem the notes pursuant to their terms.
 
                                      S-14
<PAGE>
                  UNITED STATES TAX DOCUMENTATION REQUIREMENTS
 
INTRODUCTION
 
    The following discussion of United States tax documentation requirements
does not deal with all aspects of United States federal income tax withholding
or reporting that may be relevant to a beneficial owner of the notes that is a
non-United States person. Investors should consult their tax advisors for
specific advice concerning the acquisition, ownership and disposition of the
notes.
 
DOCUMENTATION REQUIRED IN ORDER TO OBTAIN AN EXEMPTION FROM WITHHOLDING TAX
 
    A 30% United States federal withholding tax will generally apply to payments
of interest on the notes, unless the beneficial owner of a note takes one of the
following steps to obtain an exemption from or reduction of the tax. The 30%
tax, however, may be allowed as a refund or credit against the beneficial
owner's United States federal income tax liability. In addition, if a beneficial
owner of a note does not properly provide the required documentation, or if such
documentation is not properly transmitted to and received by the United States
person required to withhold United States federal income tax, the beneficial
owner will not be entitled to any additional amounts from Citigroup described
under "Description of Notes--Payment of Additional Amounts" above.
 
(1) NON-UNITED STATES PERSONS: IRS FORM W-8. A beneficial owner of a note that
    is a non-United States person can obtain an exemption from the withholding
    tax by providing a properly completed IRS Form W-8, titled "Certificate of
    Foreign Status." This exemption is not available to:
 
    -  a controlled foreign corporation that is directly or indirectly related
       to Citigroup through stock ownership;
 
    -  a person that actually or constructively owns 10 percent or more of the
       total combined voting power of all classes of stock of Citigroup that are
       entitled to vote; or
 
    -  a bank that has invested in the note as an extension of credit in the
       ordinary course of its trade or business.
 
(2) NON-UNITED STATES PERSONS WITH EFFECTIVELY CONNECTED INCOME: IRS FORM 4224.
    A beneficial owner of a note that is a non-United States person, including a
    non-United States corporation or bank with a United States branch, that
    conducts a trade or business in the United States with which the interest
    income on a note is effectively connected, can obtain an exemption from the
    withholding tax by providing a properly completed IRS Form 4224, titled
    "Exemption from Withholding of Tax on Income Effectively Connected with the
    Conduct of a Trade or Business in the United States."
 
(3) NON-UNITED STATES PERSONS ENTITLED TO INCOME TAX TREATY BENEFITS: IRS FORM
    1001. A beneficial owner of a note that is a non-United States person
    entitled to the benefits of an income tax treaty to that the United States
    is a party can obtain an exemption from or reduction of the withholding tax
    by providing a properly completed IRS Form 1001, titled "Ownership,
    Exemption or Reduced Rate Certificate." The availability and extent of such
    exemption, however, will depend upon the terms of the particular income tax
    treaty.
 
(4) UNITED STATES PERSONS: IRS FORM W-9. A beneficial owner of a note that is a
    United States person can obtain an exemption from the withholding tax by
    providing a properly completed IRS Form W-9, titled "Request for Taxpayer
    Identification Number and Certification."
 
UNITED STATES FEDERAL INCOME TAX REPORTING PROCEDURE
 
    BENEFICIAL OWNERS.  A beneficial owner of a note is required to submit the
appropriate IRS form under applicable procedures to the person through which the
owner directly holds the note. For
 
                                      S-15
<PAGE>
example, if the beneficial owner is listed directly on the books of Euroclear or
Cedelbank as the owner of the note, the IRS form must be provided to Euroclear
or Cedelbank, as the case may be.
 
    OTHER PERSONS THROUGH WHICH A NOTE IS HELD.  Each other person through which
a note is held must submit, on behalf of the beneficial owner, the IRS form, or
in some cases a copy of such form, under applicable procedures through the chain
of intermediaries, until the IRS form is received by the United States person
that would be required to withhold United States federal income tax from
interest on the note. For example, in the case of a note held through Euroclear
or Cedelbank, the IRS form, or a copy of such form, must be received by the U.S.
depositary of such clearing agency.
 
    APPLICABLE PROCEDURES.  Applicable procedures include additional
certification requirements if a beneficial owner of the note provides an IRS
Form W-8 to a securities clearing organization, bank or other financial
institution that holds the note on its behalf.
 
    NEW REGULATIONS APPLICABLE TO PAYMENTS MADE AFTER DECEMBER 31,
1999.  Regulations recently issued by the IRS, which will be effective for
payments made after December 31, 1999, make modifications to the certification
procedures applicable to non-United States persons. In particular, revised IRS
forms have been issued that will replace some of the tax forms described above,
and beneficial owners may be requested to provide these revised forms in
addition to, or instead of, the existing IRS forms. In addition, the new
regulations address various issues relating to financial intermediary
certification procedures and also require, in the case of notes held by a
foreign partnership, that IRS forms be provided by the partners rather than the
partnership and that the partnership provide particular information, including a
United States taxpayer identification number. Prospective investors should
consult their tax advisors regarding the certification requirements for
non-United States persons under the new regulations.
 
                                      S-16
<PAGE>
                UNITED STATES FEDERAL INCOME TAX CONSIDERATIONS
                         FOR NON-UNITED STATES HOLDERS
 
INTRODUCTION
 
    The following is a general summary of United States federal income tax
considerations that may be relevant to a beneficial owner of notes that is a
non-United States person (a "non-United States holder"). The summary is based on
laws, regulations, rulings and decisions now in effect, all of which may change,
possibly with retroactive effect. This summary deals only with beneficial owners
that will hold notes as capital assets.
 
    This summary does not address all of the United States federal income tax
considerations that may be relevant to non-United States holders. For example,
this summary does not address tax considerations applicable to investors to
which special tax rules may apply, including:
 
    -  banks;
 
    -  tax-exempt entities;
 
    -  insurance companies;
 
    -  regulated investment companies;
 
    -  common trust funds;
 
    -  dealers in securities or currencies;
 
    -  persons that will hold the notes as a part of an integrated investment,
       including a "straddle" or "conversion transaction," comprised of a note
       and one or more other positions; or
 
    -  persons that have a "functional currency" other than the U.S. dollar.
 
    Prospective investors should consult their tax advisors in determining the
particular United States federal income tax consequences to them of the
acquisition, ownership and disposition of the notes, as well as the application
of state, local, foreign or other tax laws.
 
INCOME AND WITHHOLDING TAX
 
INTEREST PAYMENTS ON A NOTE
 
    Withholding of United States federal income tax will not apply to interest
payments on a note to a non-United States holder so long as:
 
    -  the holder does not actually or constructively own 10 percent or more of
       the total combined voting power of all classes of stock of Citigroup
       entitled to vote and is not a controlled foreign corporation that is
       directly or indirectly related to Citigroup through stock ownership; and
 
    -  the beneficial owner provides a statement on IRS Form W-8 or any
       successor form signed under penalties of perjury that includes its name
       and address and certifies that it is a non-United States person in
       compliance with applicable requirements, or, in the case of payments made
       after December 31, 1999, satisfies documentary evidence requirements for
       establishing that it is a non-United States person.
 
    For further details, see "United States Tax Documentation
Requirements--Documentation Required in Order to Obtain an Exemption from
Withholding Tax--(1) Non-United States Persons: IRS Form W-8."
 
GAIN REALIZED ON THE SALE, EXCHANGE OR OTHER TAXABLE DISPOSITION OF A NOTE
 
    Withholding of United States federal income tax will not apply to gain
realized on the sale, exchange, or other taxable disposition of a note by a
non-United States holder.
 
                                      S-17
<PAGE>
    NON-UNITED STATES HOLDERS THAT ARE TAXABLE ON A NET INCOME BASIS
 
    The rules described above under "Interest Payments on a Note" and "Gain
Realized on the Sale, Exchange or Other Taxable Disposition of a Note" will not
apply to a non-United States holder to which United States federal income
taxation applies on a net income basis. Rather, such non-United States holder
generally will be taxable on interest payments received on a note and gain
recognized upon the sale, exchange or other taxable disposition of a note under
the same rules which govern the taxation of United States persons. Special rules
might also apply to a non-United States holder that is a qualified resident of a
country with which the United States has an income tax treaty.
 
BACKUP WITHHOLDING AND INFORMATION REPORTING
 
    INTEREST PAYMENTS ON A NOTE
 
    United States information reporting requirements and backup withholding tax
will not apply to interest payments on a note if the beneficial owner certifies
on IRS Form W-8 or any successor form that it is a non-United States person
under penalties of perjury, or, in the case of a payment made after December 31,
1999, satisfies documentary evidence requirements for establishing that is a
non-United States person, or otherwise establishes an exemption.
 
    PROCEEDS OF THE SALE, EXCHANGE OR OTHER TAXABLE DISPOSITION OF A NOTE
 
    Information reporting requirements and backup withholding tax will not apply
to a payment of the proceeds of the sale, exchange or other taxable disposition
of a note effected outside the United States by a foreign office of a foreign
broker, as defined in applicable Treasury regulations, provided that such
broker:
 
    -  derives less than 50% of its gross income for particular periods from the
       conduct of a trade or business in the United States;
 
    -  is not a controlled foreign corporation for United States federal income
       tax purposes; and
 
    -  for payments made after December 31, 1999, is not a foreign partnership
       that, at any time during its taxable year is 50% or more, by income or
       capital interest, owned by United States persons or is engaged in the
       conduct of a United States trade or business.
 
    Information reporting requirements, but not backup withholding tax, will
apply to a payment of the proceeds of the sale, exchange, or other taxable
disposition of a note effected outside the United States by a foreign office of
any other broker, unless such broker has documentary evidence in its records
that the beneficial owner is a non-United States person and other conditions are
met, or the beneficial owner otherwise establishes an exemption.
 
    Information reporting requirements and backup withholding tax will apply to
a payment of the proceeds of the sale, exchange or other taxable disposition of
a note by the United States office of a broker unless the beneficial owner
certifies that it is a non-United States person under penalties of perjury or
otherwise establishes an exemption.
 
    NEW REGULATIONS APPLICABLE TO PAYMENTS AFTER DECEMBER 31, 1999
 
    The United States Treasury Department recently issued final Treasury
regulations governing information reporting and the certification procedures
regarding withholding and backup withholding on amounts paid to non-United
States persons after December 31, 1999. See "United States Tax Documentation
Requirements--United States Federal Income Tax Reporting Procedure--New
Regulations Applicable to Payments Made After December 31, 1999."
 
                                      S-18
<PAGE>
                                  UNDERWRITING
 
    The terms and conditions set forth in the terms agreements dated March 26,
1999, each of which incorporates by reference the underwriting agreement basic
provisions dated January 12, 1993, govern the sale and purchase of the notes.
The terms agreements and underwriting agreement basic provisions are referred to
together as the underwriting agreement. Each underwriter named below has
severally agreed to purchase from Citigroup, and Citigroup has agreed to sell to
each underwriter, the number of notes set forth opposite the name of each
underwriter.
 
<TABLE>
<CAPTION>
                                                                               PRINCIPAL AMOUNT  PRINCIPAL AMOUNT
                                 UNDERWRITER                                    OF 2004 NOTES     OF 2009 NOTES
                       ------------------------------                          ----------------  ----------------
<S>                                                                            <C>               <C>
Salomon Smith Barney Inc.....................................................   $  640,000,000    $  650,000,000
ABN AMRO Incorporated........................................................        5,000,000        20,000,000
Barclays Bank PLC............................................................        5,000,000                --
BBV Securities, Inc..........................................................        5,000,000                --
Bear, Stearns & Co. Inc......................................................               --        20,000,000
Chase Securities Inc.........................................................       15,000,000                --
Credit Suisse First Boston Corporation.......................................       15,000,000                --
Commerzbank Aktiengesellschaft...............................................        5,000,000                --
Donaldson, Lufkin & Jenrette Securities Corporation..........................       15,000,000                --
First Chicago Capital Markets, Inc...........................................        5,000,000                --
First Union Capital Markets Corp.............................................        5,000,000                --
Goldman, Sachs & Co..........................................................               --        20,000,000
Lehman Brothers Inc..........................................................       15,000,000                --
Merrill Lynch, Pierce, Fenner & Smith
           Incorporated......................................................               --        20,000,000
NationsBanc Montgomery Securities LLC........................................               --        20,000,000
Rabobank International.......................................................        5,000,000                --
Societe Generale.............................................................       15,000,000                --
                                                                               ----------------  ----------------
    Total....................................................................   $  750,000,000    $  750,000,000
                                                                               ----------------  ----------------
                                                                               ----------------  ----------------
</TABLE>
 
    The underwriting agreement provides that the obligations of the underwriters
to pay for and accept delivery of the notes are subject to the approval of legal
matters by their counsel and to other conditions. The underwriters are committed
to take and pay for all of the notes if any are taken.
 
    The underwriters propose to offer part of the notes directly to the public
at the public offering prices set forth on the cover page of this prospectus
supplement and to certain dealers at the public offering price less a concession
not in excess of:
 
    -  0.250% of the principal amount in the case of the 2004 notes; and
 
    -  0.350% of the principal amount in the case of the 2009 notes.
 
    The underwriters may allow, and such dealers may reallow, a concession to
certain other dealers not in excess of:
 
    -  0.125% of the principal amount in the case of the 2004 notes; and
 
    -  0.250% of the principal amount in the case of the 2009 notes.
 
    After the public offering, the public offering prices and the concessions to
dealers may be changed by the underwriters.
 
    The underwriters are offering the notes subject to prior sale and their
acceptance of the notes from Citigroup. The underwriters may reject any order in
whole or in part.
 
                                      S-19
<PAGE>
    Citigroup has agreed to indemnify the underwriters against liabilities
relating to material misstatements and omissions.
 
    In accordance with Regulation M of the Securities Exchange Act of 1934, the
underwriters may over-allot or effect transactions that stabilize or cover, each
of which is described below.
 
    -  Over-allotment involves sales in excess of the offering size, which
       creates a short position for the underwriters.
 
    -  Stabilizing transactions involve bids to purchase the underlying security
       so long as the stabilizing bids do not exceed a specified maximum.
 
    -  Covering transactions involve purchases of the notes in the open market
       after the distribution has been completed in order to cover short
       positions.
 
    These transactions may cause the price of the notes to be higher than it
would otherwise be in the absence of such transactions. The underwriters are not
required to engage in any of these activities and may end any of these
activities at any time. The underwriters may also impose a penalty bid. Penalty
bids permit an underwriter to reclaim a selling concession from a syndicate
member when that underwriter, in covering syndicate short positions or making
stabilizing purchases, purchases notes originally sold by that syndicate member.
 
    The notes are new issues of securities with no established trading market.
Citigroup has applied for listing of the notes on the Luxembourg Stock Exchange.
Citigroup has been advised by the underwriters that they presently intend to
make a market in the notes, as permitted by applicable laws and regulations. The
underwriters are not obligated, however, to make a market in the notes and may
discontinue any market making at any time at their sole discretion. Accordingly,
Citigroup can make no assurance as to the liquidity of, or trading markets for,
the notes.
 
    The underwriters and their affiliates may engage in transactions (which may
include commercial banking transactions) with, and perform services for,
Citigroup or one or more of its affiliates in the ordinary course of business.
 
    The broker-dealer subsidiaries of Citigroup are members of the National
Association of Securities Dealers, Inc. and one or more of such broker-dealer
subsidiaries will participate in the offering of the notes. Accordingly, the
offering of the notes will conform with the requirements set forth in Rule 2720
of the Conduct Rules of the NASD.
 
    This prospectus supplement, together with the accompanying prospectus, may
also be used by Citigroup's broker-dealer subsidiaries or other subsidiaries or
affiliates of Citigroup in connection with offers and sales of the notes in
market-making transactions at negotiated prices related to prevailing market
prices at the time of sale. Any of these subsidiaries may act as principal or
agent in such transactions.
 
    The notes are being offered globally for sale in the United States, Europe,
Asia and elsewhere where it is lawful to make such offers.
 
    Purchasers of the notes may be required to pay stamp taxes and other charges
in accordance with the laws and practices of the country of purchase in addition
to the issue price set forth on the cover page of this document.
 
    The underwriters have agreed that they will not offer, sell or deliver any
of the notes, directly or indirectly, or distribute this prospectus supplement
or the accompanying prospectus or any other offering material relating to the
notes, in or from any jurisdiction, except when to the best knowledge and belief
of the underwriters it is permitted under applicable laws and regulations. In so
doing, the underwriters will not impose any obligations on Citigroup, except as
set forth in the underwriting agreement.
 
                                      S-20
<PAGE>
    Each underwriter has represented and agreed that:
 
    -  it has not offered or sold, and, prior to the expiration of the period of
       six months from the closing date for the issuance of the notes, will not
       offer or sell any notes to persons in the United Kingdom, except to those
       persons whose ordinary activities involve them in acquiring, holding,
       managing or disposing of investments, as principal or agent, for the
       purposes of their businesses or otherwise in circumstances that have not
       resulted and will not result in an offer to the public in the United
       Kingdom within the meaning of the Public Offers of Securities Regulations
       1995;
 
    -  it has complied and will comply with all applicable provisions of the
       Financial Services Act 1986 with respect to anything done by it in
       relation to the notes in, from or otherwise involving the United Kingdom;
 
    -  it has only issued or passed on and will only issue or pass on in the
       United Kingdom any document received by it in connection with the issue
       or sale of the notes to a person who is of a kind described in Article
       11(3) of the Financial Services Act 1986 (Investment Advertisements)
       (Exemptions) Order 1996, as amended, or is a person to whom the document
       may otherwise lawfully be issued or passed on;
 
    -  it will not offer or sell any notes directly or indirectly in Japan or
       to, or for the benefit of any Japanese person or to others, for
       re-offering or re-sale directly or indirectly in Japan or to any Japanese
       person except under circumstances which will result in compliance with
       all applicable laws, regulations and guidelines promulgated by the
       relevant governmental and regulatory authorities in effect at the
       relevant time. For purposes of this paragraph, "Japanese person" means
       any person resident in Japan, including any corporation or other entity
       organized under the laws of Japan;
 
    -  it is aware of the fact that no German selling prospectus
       (VERKAUFSPROSPEKT) has been or will be published in respect of the sale
       of the notes and that it will comply with the Securities Selling
       Prospectus Act (the "SSPA") of the Federal Republic of Germany
       (WERTPAPIER-VERKAUFSPROSPEKTGESETZ). In particular, each underwriter has
       undertaken not to engage in a public offering (OFFENTLICHE ANBIETEN) in
       the Federal Republic of Germany with respect to any notes otherwise than
       in accordance with the SSPA and any other act replacing or supplementing
       the SSPA and all other applicable laws and regulations;
 
    -  the notes are being issued and sold outside the Republic of France and
       that, in connection with their initial distribution, it has not offered
       or sold and will not offer or sell, directly or indirectly, any notes to
       the public in the Republic of France, and that it has not distributed and
       will not distribute or cause to be distributed to the public in the
       Republic of France this prospectus supplement, the accompanying
       prospectus or any other offering material relating to the notes; and
 
    -  it and each of its affiliates have not offered or sold, and will not
       offer or sell, the notes by means of any document to persons in Hong Kong
       other than persons whose ordinary business it is to buy or sell shares or
       debentures, whether as principal or agent, or otherwise in circumstances
       which do not constitute an offer to the public within the meaning of the
       Hong Kong Companies Ordinance (Chapter 32 of the Laws of Hong Kong).
 
                          NOTICE TO CANADIAN RESIDENTS
 
RESALE RESTRICTIONS
 
    The distribution of the notes in Canada is being made only on a private
placement basis exempt from the requirement that Citigroup prepare and file a
prospectus with the securities regulatory
 
                                      S-21
<PAGE>
authorities in each province where trades of the notes are effected.
Accordingly, any resale of the notes in Canada must be made in accordance with
applicable securities laws which will vary depending on the relevant
jurisdiction, and which may require resales to be made in accordance with
available statutory exemptions or pursuant to a discretionary exemption granted
by the applicable Canadian securities regulatory authority. Purchasers are
advised to seek legal advice prior to any resale of the notes.
 
REPRESENTATIONS OF PURCHASERS
 
    Each purchaser of notes in Canada who receives a purchase confirmation will
be deemed to represent to Citigroup and the dealer from whom such purchase
confirmation is received that (1) such purchaser is entitled under applicable
provincial securities laws to purchase such notes without the benefit of a
prospectus qualified under such securities laws or an offering memorandum, (2)
where required by law, that such purchaser is purchasing as principal and not as
agent, (3) if in Quebec, such purchaser shall be deemed to have agreed that all
documents relating to such purchase be in English only, and (4) such purchaser
has reviewed the terms above under "Resale Restrictions."
 
RIGHTS OF ACTION AND ENFORCEMENT
 
    The securities being offered are those of a foreign issuer and Ontario
purchasers will not receive the contractual right of action prescribed by
section 32 of the Regulation under the Securities Act (Ontario). As a result,
Ontario purchasers must rely on other remedies that may be available, including
common law rights of action for damages or rescission or rights of action under
the civil liability provisions of the U.S. federal securities laws.
 
    All of Citigroup's directors and officers as well as the experts named
herein may be located outside of Canada and, as a result, it may not be possible
for Ontario purchasers to effect service of process within Canada on Citigroup
or such persons. All or a substantial portion of the assets of Citigroup and
such persons may be located outside of Canada and, as a result, it may not be
possible to satisfy a judgment against Citigroup or such persons in Canada or to
enforce a judgment obtained in Canadian courts against Citigroup or such persons
outside of Canada.
 
NOTICE TO BRITISH COLUMBIA RESIDENTS
 
    A purchaser of notes to whom the Securities Act (British Columbia) applies
is advised that such purchaser is required to file with the British Columbia
Securities Commission a report within ten days of the sale of any notes acquired
by such purchaser pursuant to this offering. Such report must be in the form
attached to British Columbia Securities Commission Blanket Order BOR No. 95/17,
a copy of which may be obtained from Citigroup. Only one such report must be
filed in respect of notes acquired on the same date and under the same
prospectus exemption.
 
               DIRECTORS AND EXECUTIVE OFFICERS OF CITIGROUP INC.
 
    The members of the board of directors of Citigroup are: C. Michael
Armstrong, Alain J.P. Belda, Kenneth J. Bialkin, Kenneth T. Derr, John M.
Deutch, Ann Dibble Jordan, Reuben Mark, Michael T. Masin, Dudley C. Mecum,
Richard D. Parsons, Andrall E. Pearson, John S. Reed, Robert B. Shapiro,
Franklin A. Thomas, Sanford I. Weill, Edgar S. Woolard, Jr. and Arthur Zankel.
The Honorable Gerald R. Ford is an honorary director of Citigroup. The executive
officers of Citigroup are: John S. Reed, Sanford I. Weill, William I. Campbell,
Michael A. Carpenter, Paul J. Collins, Edward D. Horowitz, Thomas W. Jones,
Robert I. Lipp, Deryck C. Maughan, Victor J. Menezes, Heidi G. Miller, Charles
O. Prince, III, Mary Alice Taylor, Todd S. Thomson and Marc P. Weill. The
business address of each director and executive officer of Citigroup in such
capacities is 153 East 53rd Street, New York, NY 10043.
 
                                      S-22
<PAGE>
                                 LEGAL OPINIONS
 
    The validity of the notes will be passed upon for Citigroup by Stephanie B.
Mudick, General Counsel--Corporate Law and an Assistant Secretary of Citigroup,
153 East 53rd Street, New York, New York 10043, and for the underwriters by
Dewey Ballantine LLP, 1301 Avenue of the Americas, New York, New York
10019-6092. Skadden, Arps, Slate, Meagher & Flom LLP, New York, New York, has
acted as special tax counsel to Citigroup in connection with tax matters related
to the issuance of the notes. Ms. Mudick beneficially owns, or has rights to
acquire under Citigroup's employee benefit plans, an aggregate of less than 1%
of Citigroup's common stock. Dewey Ballantine LLP has from time to time acted as
counsel for Citigroup and its subsidiaries and may do so in the future. A member
of Dewey Ballantine LLP participating in this matter is the beneficial owner of
an aggregate of less than 1% of Citigroup's common stock. Kenneth J. Bialkin, a
partner of Skadden, Arps, Slate, Meagher & Flom LLP, is a director of Citigroup
and he and other attorneys in his firm beneficially own an aggregate of less
than 1% of the common stock of Citigroup.
 
                              GENERAL INFORMATION
 
    Application will be made to list the notes on the Luxembourg Stock Exchange.
In connection with the listing application, the certificate of incorporation and
the by-laws of Citigroup and a legal notice relating to the issuance of the
notes will be deposited prior to listing with GREFFIER EN CHEF DU TRIBUNAL
D'ARRONDISSEMENT DE ET A LUXEMBOURG. You may request copies of these documents
together with this prospectus supplement, the accompanying prospectus, the
underwriting agreement, the indenture and Citigroup's current annual and
quarterly reports, as well as all other documents incorporated by reference in
this prospectus supplement, including future annual and quarterly reports, so
long as any of the notes are outstanding.
 
    These documents also will be made available for inspection at the main
office of Banque Internationale a Luxembourg S.A. in Luxembourg. Banque
Internationale a Luxembourg S.A. will act as intermediary between the Luxembourg
Stock Exchange and Citigroup and the holders of the notes.
 
    You can also request copies of (1) this prospectus supplement, the
accompanying prospectus and the indenture, and (2) Citigroup's annual, quarterly
and current reports, as well as other documents incorporated by reference in
this prospectus supplement, including future annual, quarterly and current
reports, by following the directions under "Where You Can Find More Information"
on page 6 of the accompanying prospectus.
 
    Neither Citigroup nor any of its subsidiaries is involved in litigation,
arbitration or administrative proceedings relating to claims or amounts that are
material in the context of the issue of the notes. Citigroup is not aware of any
such litigation, arbitration or administrative proceedings pending or
threatened.
 
    Citigroup accepts responsibility for the information contained in this
prospectus supplement and the accompanying prospectus.
 
    Resolutions relating to the issue and sale of the notes were adopted by the
board of directors of Citigroup on December 15, 1998 and by the Notes Committee
of the board of directors on March 26, 1999.
 
    The 2004 notes have been assigned Euroclear and Cedelbank Common Code No.
9626018, International Security Identification Number (ISIN) US172967AW18 and
CUSIP No. 172967AW1. The 2009 notes have been assigned Euroclear and Cedelbank
Common Code No. 9626077, International Security Identification Number (ISIN)
US172967AX90 and CUSIP No. 172967AX9.
 
                                      S-23
<PAGE>
PROSPECTUS
 
                                     [LOGO]
 
May Offer--
 
                                 $6,000,000,000
                                Debt Securities
                                 Index Warrants
                                Preferred Stock
                               Depositary Shares
                                  Common Stock
 
    Citigroup will provide the specific terms of these securities in supplements
to this prospectus. You should read this prospectus and the accompanying
prospectus supplement carefully before you invest.
 
                            ------------------------
 
    Neither the Securities and Exchange Commission nor any state securities or
insurance commission has approved or disapproved of these securities or
determined if this prospectus or any accompanying prospectus supplement is
truthful or complete. Any representation to the contrary is a criminal offense.
 
    These securities are not deposits or savings accounts but are unsecured
obligations of Citigroup Inc. These securities are not insured by the Federal
Deposit Insurance Corporation or any other governmental agency or
instrumentality.
 
                            ------------------------
 
March 8, 1999
<PAGE>
                               PROSPECTUS SUMMARY
 
    This summary provides a brief overview of the key aspects of Citigroup and
all material terms of the offered securities which are known as of the date of
this prospectus. For a more complete understanding of the terms of the offered
securities, before making your investment decision, you should carefully read:
 
    - this prospectus, which explains the general terms of the securities that
      Citigroup may offer;
 
    - the accompanying prospectus supplement, which (1) explains the specific
      terms of the securities being offered and (2) updates and changes
      information in this prospectus; and
 
    - the documents referred to in "Where You Can Find More Information" on page
      6 for information on Citigroup, including its financial statements.
 
                                 CITIGROUP INC.
 
    Citigroup's businesses provide a broad range of financial services to
consumer and corporate customers around the world. Among these businesses are
Citibank, Commercial Credit, Primerica Financial Services, Salomon Smith Barney,
SSB Citi Asset Management, Travelers Life & Annuity and Travelers Property
Casualty.
 
    On October 8, 1998, Citigroup changed its name from Travelers Group Inc. to
Citigroup Inc. in connection with the merger of Citicorp into a subsidiary of
Citigroup. Citigroup's principal executive office is at 153 East 53(rd) Street,
New York, NY 10043, and its telephone number is (212) 559-1000.
 
                       THE SECURITIES CITIGROUP MAY OFFER
 
    Citigroup may use this prospectus to offer up to $6,000,000,000 of:
 
    - debt securities;
 
    - index warrants;
 
    - preferred stock;
 
    - depositary shares; and
 
    - common stock.
 
    A prospectus supplement will describe the specific types, amounts, prices,
and detailed terms of any of these offered securities.
 
DEBT SECURITIES
 
    Debt securities are unsecured general obligations of Citigroup in the form
of senior or subordinated debt. Senior debt includes Citigroup's notes, debt and
guarantees and any other debt for money borrowed that is not subordinated.
Subordinated debt, so designated at the time it is issued, would not be entitled
to interest and principal payments if payments on the senior debt were not made.
 
    The senior and subordinated debt will be issued under separate indentures
between Citigroup and a trustee. Below are summaries of the general features of
the debt securities from these indentures. For a more detailed description of
these features, see "Description of Debt Securities" below. You are also
encouraged to read the indentures, which are incorporated by reference in or
filed as exhibits to Citigroup's registration statement No. 333-68949,
Citigroup's most recent annual report on Form 10-K, Citigroup's quarterly
reports on Form 10-Q filed after the Form 10-K and Citigroup's current reports
on Form 8-K filed after the Form 10-K. You can receive copies of these documents
by following the directions on page 6.
 
                                       2
<PAGE>
    GENERAL INDENTURE PROVISIONS THAT APPLY TO SENIOR AND SUBORDINATED DEBT
 
    - Neither indenture limits the amount of debt that Citigroup may issue or
      provides holders any protection should there be a highly leveraged
      transaction involving Citigroup, although the senior debt indenture does
      limit Citigroup's ability to pledge the stock of any subsidiary that meets
      the financial thresholds in the indenture. These thresholds are described
      below under "Description of Debt Securities."
 
    - Each indenture allows for different types of debt securities, including
      indexed securities, to be issued in series.
 
    - The indentures allow Citigroup to merge or to consolidate with another
      company, or sell all or substantially all of its assets to another
      company. If any of these events occur, the other company would be required
      to assume Citigroup's responsibilities for the debt. Unless the
      transaction resulted in an event of default, Citigroup would be released
      from all liabilities and obligations under the debt securities when the
      other company assumed its responsibilities.
 
    - The indentures provide that holders of 66 2/3% of the principal amount of
      the senior debt securities and holders of a majority of the total
      principal amount of the subordinated debt securities outstanding in any
      series may vote to change Citigroup's obligations or your rights
      concerning those securities. However, changes to the financial terms of
      that security, including changes in the payment of principal or interest
      on that security or the currency of payment, cannot be made unless every
      holder of that security consents to the change.
 
    - Citigroup may satisfy its obligations under the debt securities or be
      released from its obligation to comply with the limitations discussed
      above at any time by depositing sufficient amounts of cash or U.S.
      government securities with the trustee to pay Citigroup's obligations
      under the particular securities when due.
 
    - The indentures govern the actions of the trustee with regard to the debt
      securities, including when the trustee is required to give notices to
      holders of the securities and when lost or stolen debt securities may be
      replaced.
 
    EVENTS OF DEFAULT
 
    The events of default specified in the indentures include:
 
    - failure to pay principal when due;
 
    - failure to pay required interest for 30 days;
 
    - failure to make a required scheduled installment payment for 30 days;
 
    - failure to perform other covenants for 90 days after notice; and
 
    - certain events of insolvency or bankruptcy, whether voluntary or not.
 
    REMEDIES
 
    If there were a default, the trustee or holders of 25% of the principal
amount of debt securities outstanding in a series could demand that the
principal be paid immediately. However, holders of a majority in principal
amount of the securities in that series could rescind that acceleration of the
debt securities.
 
INDEX WARRANTS
 
    Citigroup may issue index warrants independently or together with debt
securities. Citigroup will issue any series of index warrants under a separate
warrant agreement between Citigroup and a bank
 
                                       3
<PAGE>
or trust company. You are encouraged to read the standard form of the warrant
agreement, which will be filed as an exhibit to one of Citigroup's future
current reports and incorporated by reference in its registration statement No.
333-68949. You can receive copies of these documents by following the directions
on page 6.
 
    Index warrants are securities that, when properly exercised by the
purchaser, entitle the purchaser to receive from Citigroup an amount in cash or
a number of securities that will be indexed to prices, yields, or other
specified measures or changes in an index or differences between two or more
indices.
 
    The prospectus supplement for a series of index warrants will describe the
formula for determining the amount in cash or number of securities, if any, that
Citigroup will pay you when you exercise an index warrant and will contain
information about the relevant underlying assets and other specific terms of the
index warrant.
 
    Citigroup will generally issue index warrants in book-entry form, which
means that they will not be evidenced by physical certificates. Also, Citigroup
will generally list index warrants for trading on a national securities
exchange, such as the New York Stock Exchange, Inc., The Nasdaq-Amex Market
Group or the Chicago Board Options Exchange, Incorporated.
 
    The warrant agreement for any series of index warrants will provide that
holders of a majority of the total principal amount of the index warrants
outstanding in any series may vote to change their rights concerning those index
warrants. However, changes to fundamental terms such as the amount or manner of
payment on an index warrant or changes to the exercise times cannot be made
unless every holder affected consents to the change.
 
    Any prospective purchasers of index warrants should be aware of special
United States federal income tax considerations applicable to instruments such
as the index warrants. The prospectus supplement relating to each series of
index warrants will describe the important tax considerations.
 
PREFERRED STOCK
 
    Citigroup may issue preferred stock with various terms to be established by
its board of directors or a committee designated by the board. Each series of
preferred stock will be more fully described in the particular prospectus
supplement that will accompany this prospectus, including redemption provisions,
rights in the event of liquidation, dissolution or winding up of Citigroup,
voting rights and conversion rights.
 
    Generally, each series of preferred stock will rank on an equal basis with
each other series of preferred stock and will rank prior to Citigroup's common
stock. The prospectus supplement will also describe how and when dividends will
be paid on the series of preferred stock.
 
DEPOSITARY SHARES
 
    Citigroup may issue depositary shares representing fractional shares of
preferred stock. Each particular series of depositary shares will be more fully
described in the prospectus supplement that will accompany this prospectus.
These depositary shares will be evidenced by depositary receipts and issued
under a deposit agreement between Citigroup and a bank or trust company. You are
encouraged to read the standard form of the deposit agreement, which is
incorporated by reference in Citigroup's registration statement No. 333-68949.
You can receive copies of this document by following the directions on page 6.
 
COMMON STOCK
 
    Citigroup may issue common stock, par value $.01 per share. Holders of
common stock are entitled to receive dividends when declared by its board of
directors. Each holder of common stock is entitled to one vote per share. The
holders of common stock have no preemptive rights or cumulative voting rights.
 
                                       4
<PAGE>
                                USE OF PROCEEDS
 
    Citigroup will use the net proceeds it receives from any offering of these
securities for general corporate purposes, primarily to fund its operating units
and subsidiaries. Citigroup may use some of the proceeds to refinance or extend
the maturity of existing debt obligations. Citigroup may use a portion of the
proceeds from the sale of index warrants and indexed notes to hedge its exposure
to payments that it may have to make on such index warrants and indexed notes as
described below under "Use of Proceeds and Hedging."
 
                              PLAN OF DISTRIBUTION
 
    Citigroup may sell the offered securities in any of the following ways:
 
    - to or through underwriters or dealers;
 
    - by itself directly;
 
    - through agents; or
 
    - through a combination of any of these methods of sale.
 
    The prospectus supplement will explain the ways Citigroup sells specific
securities, including the names of any underwriters and details of the pricing
of the securities, as well as the commissions, concessions or discounts
Citigroup is granting the underwriters, dealers or agents.
 
    If Citigroup uses underwriters in any sale, the underwriters will buy the
securities for their own account and may resell the securities from time to time
in one or more transactions, at a fixed public offering price or at varying
prices determined at the time of sale. In connection with an offering,
underwriters and selling group members and their affiliates may engage in
transactions to stabilize, maintain or otherwise affect the market price of the
securities, in accordance with applicable law.
 
    Citigroup expects that the underwriters for any offering will include one or
more of its broker-dealer subsidiaries, including Salomon Smith Barney Inc.
These broker-dealer subsidiaries, including their successors, also expect to
offer and sell previously issued offered securities as part of their business,
and may act as a principal or agent in such transactions. Citigroup or any of
its subsidiaries may use this prospectus and the related prospectus supplements
and pricing supplements in connection with these activities.
 
                                       5
<PAGE>
                      WHERE YOU CAN FIND MORE INFORMATION
 
    As required by the Securities Act of 1933, Citigroup filed a registration
statement (No. 333-68949) relating to the securities offered by this prospectus
with the Securities and Exchange Commission. This prospectus is a part of that
registration statement, which includes additional information.
 
    Citigroup files annual, quarterly and current reports, proxy statements and
other information with the SEC. You may read and copy any document Citigroup
files at the SEC's public reference rooms in Washington, D.C., New York, New
York and Chicago, Illinois. You can also request copies of the documents, upon
payment of a duplicating fee, by writing the Public Reference Section of the
SEC. Please call the SEC at 1-800-SEC-0330 for further information on the public
reference rooms. These SEC filings are also available to the public from the
SEC's web site at http://www.sec.gov.
 
    The SEC allows Citigroup to "incorporate by reference" the information it
files with the SEC, which means that it can disclose important information to
you by referring you to those documents. The information incorporated by
reference is considered to be part of this prospectus. Information that
Citigroup files later with the SEC will automatically update information in this
prospectus. In all cases, you should rely on the later information over
different information included in this prospectus or the prospectus supplement.
Citigroup incorporates by reference the documents listed below and any future
filings made with the SEC under Section 13(a), 13(c), 14, or 15(d) of the
Securities Exchange Act of 1934:
 
    (a) Annual Report on Form 10-K for the year ended December 31, 1998;
 
    (b) Current Report on Form 8-K dated January 25, 1999; and
 
    (c) Registration Statement on Form 8-B, dated May 10, 1988, describing our
       common stock, including any amendments or reports filed for the purpose
       of updating such description.
 
    All documents Citigroup files pursuant to Section 13(a), 13(c), 14 or 15(d)
of the Exchange Act after the date of this prospectus and before the later of
(1) the completion of the offering of the securities described in this
prospectus and (2) the date the broker-dealer subsidiaries of Citigroup stop
offering securities pursuant to this prospectus shall be incorporated by
reference in this prospectus from the date of filing of such documents.
 
    You may request a copy of these filings, at no cost, by writing or
telephoning Citigroup at the following address:
 
    Treasurer
    Citigroup Inc.
    153 East 53(rd) Street
    New York, NY 10043
    212-559-1000
 
    You should rely only on the information provided in this prospectus and the
prospectus supplement, as well as the information incorporated by reference.
Citigroup has not authorized anyone to provide you with different information.
Citigroup is not making an offer of these securities in any state where the
offer is not permitted. You should not assume that the information in this
prospectus, the prospectus supplement or any documents incorporated by reference
is accurate as of any date other than the date on the front of the applicable
document.
 
                                       6
<PAGE>
                                 CITIGROUP INC.
 
    Citigroup's businesses provide a broad range of financial services to
consumer and corporate customers around the world. Among these businesses are
Citibank, Commercial Credit, Primerica Financial Services, Salomon Smith Barney,
SSB Citi Asset Management, Travelers Life & Annuity and Travelers Property
Casualty.
 
    On October 8, 1998, Citigroup changed its name from Travelers Group Inc. to
Citigroup Inc. in connection with the merger of Citicorp into a subsidiary of
Citigroup.
 
    Citigroup is a holding company and services its obligations primarily with
dividends and advances that it receives from subsidiaries. Citigroup's
subsidiaries that operate in the banking, insurance and securities business can
only pay dividends if they are in compliance with the applicable regulatory
requirements imposed on them by federal bank regulatory authorities, state
insurance departments and securities regulators. Citigroup's subsidiaries may be
party to credit agreements that also may restrict their ability to pay
dividends. Citigroup currently believes that none of these regulatory or
contractual restrictions on the ability of its subsidiaries to pay dividends
will affect Citigroup's ability to service its own debt. Citigroup must also
maintain the required capital levels of a bank holding company before it may pay
dividends on its stock. Each of Citigroup's major operating subsidiaries
finances its operations on a stand-alone basis consistent with its
capitalization and ratings.
 
    Under longstanding policy of The Board of Governors of the Federal Reserve
System, a bank holding company is expected to act as a source of financial
strength for its subsidiary banks and to commit resources to support such banks.
As a result of that policy, Citigroup may be required to commit resources to its
subsidiary banks.
 
    The principal office of Citigroup is located at 153 East 53rd Street, New
York, NY 10043, and its telephone number is (212) 559-1000.
 
                                       7
<PAGE>
                          USE OF PROCEEDS AND HEDGING
 
    GENERAL.  Citigroup will use the proceeds it receives from the sale of the
offered securities for general corporate purposes, principally to:
 
    - fund the business of its operating units;
 
    - fund investments in, or extensions of credit or capital contributions to,
      its subsidiaries; and
 
    - lengthen the average maturity of liabilities, which means that it could
      reduce its short-term liabilities or refund maturing indebtedness.
 
    Citigroup expects to incur additional indebtedness in the future to fund its
businesses. Citigroup or an affiliate may enter into a swap agreement in
connection with the sale of the offered securities and may earn additional
income from that transaction.
 
    USE OF PROCEEDS RELATING TO INDEX WARRANTS AND INDEXED NOTES.  Citigroup or
one or more of its subsidiaries may use all or some of the proceeds received
from the sale of index warrants or indexed notes to purchase or maintain
positions in the assets that are used to determine the relevant index or
indices. Citigroup or one or more of its subsidiaries may also purchase or
maintain positions in options, futures contracts, forward contracts or swaps, or
options on such securities, or other derivative or similar instruments relating
to the relevant index or underlying assets. Citigroup may also use the proceeds
to pay the costs and expenses of hedging any currency, interest rate or other
index-related risk relating to such index warrants and indexed notes.
 
    Citigroup expects that it or one or more of its subsidiaries will increase
or decrease their initial hedging position over time using techniques which help
evaluate the size of any hedge based upon a variety of factors affecting the
value of the underlying instrument. These factors may include the history of
price changes in that underlying instrument and the time remaining to maturity.
Citigroup may take long or short positions in the index, the underlying assets,
options, futures contracts, forward contracts, swaps, or other derivative or
similar instruments related to the index and the underlying assets. These other
hedging activities may occur from time to time before the index warrants and
indexed notes mature and will depend on market conditions and the value of the
index and the underlying assets.
 
    In addition, Citigroup or one or more of its subsidiaries may purchase or
otherwise acquire a long or short position in index warrants and indexed notes
from time to time and may, in their sole discretion, hold, resell, exercise,
cancel or retire such offered securities. Citigroup or one or more of its
subsidiaries may also take hedging positions in other types of appropriate
financial instruments that may become available in the future.
 
    If Citigroup or one or more of its subsidiaries has a long hedge position
in, options contracts in, or other derivative or similar instruments related to,
the underlying assets or index, Citigroup or one or more of its subsidiaries may
liquidate all or a portion of its holdings at or about the time of the maturity
of the index warrants and indexed notes. The aggregate amount and type of such
positions are likely to vary over time depending on future market conditions and
other factors. Citigroup is only able to determine profits or losses from any
such position when the position is closed out and any offsetting position or
positions are taken into account.
 
    Citigroup has no reason to believe that its hedging activity will have a
material impact on the price of such options, swaps, futures contracts, options
on the foregoing, or other derivative or similar instruments, or on the value of
the index or the underlying assets. However, Citigroup cannot guarantee you that
its hedging activities will not affect such prices or value. Citigroup will use
the remainder of the proceeds from the sale of index warrants and indexed notes
for the general corporate purposes described above.
 
                                       8
<PAGE>
                      RATIO OF INCOME TO FIXED CHARGES AND
                   RATIO OF INCOME TO COMBINED FIXED CHARGES
                      INCLUDING PREFERRED STOCK DIVIDENDS
 
    The following table shows (1) the consolidated ratio of income to fixed
charges and (2) the consolidated ratio of income to combined fixed charges
including preferred stock dividends of Citigroup for each of the five most
recent fiscal years.
<TABLE>
<CAPTION>
                                                                                                YEAR ENDED
                                                                                               DECEMBER 31,
                                                                                ------------------------------------------
                                                                                  1998       1997       1996       1995
                                                                                ---------  ---------  ---------  ---------
<S>                                                                             <C>        <C>        <C>        <C>
Ratio of income to fixed charges (excluding interest on deposits).............       1.57       1.71       1.88       1.65
Ratio of income to fixed charges (including interest on deposits).............       1.33       1.43       1.51       1.39
Ratio of income to combined fixed charges including preferred stock dividends
  (excluding interest on deposits)............................................       1.54       1.66       1.80       1.56
Ratio of income to combined fixed charges including preferred stock dividends
  (including interest on deposits)............................................       1.32       1.41       1.48       1.35
 
<CAPTION>
 
                                                                                  1994
                                                                                ---------
<S>                                                                             <C>
Ratio of income to fixed charges (excluding interest on deposits).............       1.41
Ratio of income to fixed charges (including interest on deposits).............       1.25
Ratio of income to combined fixed charges including preferred stock dividends
  (excluding interest on deposits)............................................       1.34
Ratio of income to combined fixed charges including preferred stock dividends
  (including interest on deposits)............................................       1.21
</TABLE>
 
                                       9
<PAGE>
                            EUROPEAN MONETARY UNION
 
    The foreign currencies in which debt securities may be denominated or
payments in respect of index warrants may be due or by which amounts due on the
offered securities may be calculated could be issued by countries participating
in Stage III of the European Economic and Monetary Union.
 
    Stage III began on January 1, 1999 for the eleven participating member
states of the European Union that satisfied the economic convergence criteria in
the Treaty on European Union: Austria, Belgium, Finland, France, Germany,
Ireland, Italy, Luxembourg, The Netherlands, Portugal and Spain. Other member
states of the European Union may still become participating member states after
January 1, 1999.
 
    Stage III includes the introduction of the Euro, which, along with the
present national currency of each participating member state, is legal tender in
the participating member states. It is currently anticipated that on and after
January 1, 2002, the national currencies of participating member states will
cease to exist and the sole legal tender in such states will be the Euro. The
European Union has adopted regulations providing specific rules for the
introduction of the Euro in substitution for the respective current national
currencies of such member states, and may adopt additional regulations or
legislation in the future relating to the Euro. It is anticipated that these
regulations or legislation will be supplemented by legislation of the individual
member states.
 
    Pursuant to European Council Regulation No. 2866/98 of December 31, 1998,
one Euro equals:
 
<TABLE>
<S>                                    <C>
- -  13.7603 Austrian schillings,        -  1,936.27 Italian lire,
- -  40.3399 Belgian francs,             -  40.3399 Luxembourg francs,
- -  5.94573 Finnish marks,              -  2.20371 Dutch guilders,
- -  6.55957 French francs,              -  200.482 Portugese escudos, or
- -  1.95583 German marks,               -  166.386 Spanish pesetas.
- -  0.787564 Irish pounds,
</TABLE>
 
                                       10
<PAGE>
                         DESCRIPTION OF DEBT SECURITIES
 
    The debt securities offered by this prospectus will be unsecured obligations
of Citigroup and will be either senior or subordinated debt. Senior debt will be
issued under a senior debt indenture. Subordinated debt will be issued under a
subordinated debt indenture. The senior debt indenture and the subordinated debt
indenture are sometimes referred to in this prospectus individually as an
"indenture" and collectively as the "indentures." Forms of the indentures have
been filed with the SEC and are incorporated by reference in or filed as
exhibits to the registration statement on Form S-3 (No. 333-68949) under the
Securities Act of 1933 of which this prospectus forms a part.
 
    The following briefly summarizes the material provisions of the indentures
and the debt securities, other than pricing and related terms disclosed in the
accompanying prospectus supplement. You should read the more detailed provisions
of the applicable indenture, including the defined terms, for provisions that
may be important to you. You should also read the particular terms of a series
of debt securities, which will be described in more detail in the applicable
prospectus supplement. Copies of the indentures may be obtained from Citigroup
or the applicable trustee. So that you may easily locate the more detailed
provisions, the numbers in parentheses below refer to sections in the applicable
indenture or, if no indenture is specified, to sections in each of the
indentures. Wherever particular sections or defined terms of the applicable
indenture are referred to, such sections or defined terms are incorporated into
this prospectus by reference, and the statement in this prospectus is qualified
by that reference.
 
    Unless otherwise provided in the applicable prospectus supplement, the
trustee under the senior debt indenture will be The Bank of New York, and the
trustee under the subordinated debt indenture will be The First National Bank of
Chicago.
 
GENERAL
 
    The indentures provide that unsecured senior or subordinated debt securities
of Citigroup may be issued in one or more series, with different terms, in each
case as authorized from time to time by Citigroup. Citigroup also has the right
to "reopen" a previous issue of a series of debt securities by issuing
additional debt securities of such series.
 
    Federal income tax consequences and other special considerations applicable
to any debt securities issued by Citigroup at a discount will be described in
the applicable prospectus supplement.
 
    Because Citigroup is a holding company, the claims of creditors of
Citigroup's subsidiaries will have a priority over Citigroup's equity rights and
the rights of Citigroup's creditors, including the holders of debt securities,
to participate in the assets of the subsidiary upon the subsidiary's
liquidation.
 
    The applicable prospectus supplement relating to any series of debt
securities will describe the following terms, where applicable:
 
    - the title of the debt securities;
 
    - whether the debt securities will be senior or subordinated debt;
 
    - the total principal amount of the debt securities;
 
    - the percentage of the principal amount at which the debt securities will
      be sold and, if applicable, the method of determining the price;
 
    - the maturity date or dates;
 
    - the interest rate or the method of computing the interest rate;
 
    - the date or dates from which any interest will accrue, or how such date or
      dates will be determined, and the interest payment date or dates and any
      related record dates;
 
                                       11
<PAGE>
    - if other than in United States dollars, the currency or currency unit in
      which payment will be made;
 
    - if the amount of any payment may be determined with reference to an index
      or formula based on a currency or currency unit other than that in which
      the debt securities are payable, the manner in which the amounts will be
      determined;
 
    - if any payments may be made at the election of Citigroup or a holder of
      debt securities in a currency or currency unit other than that in which
      the debt securities are stated to be payable, the periods within which and
      the terms upon which such election may be made;
 
    - the location where payments on the debt securities will be made;
 
    - the terms and conditions on which the debt securities may be redeemed at
      the option of Citigroup;
 
    - any obligation of Citigroup to redeem, purchase or repay the debt
      securities at the option of a holder upon the happening of any event and
      the terms and conditions of redemption, purchase or repayment;
 
    - if other than the principal amount, the portion of the principal amount of
      the debt securities payable if the maturity is accelerated;
 
    - any provisions for the discharge of Citigroup's obligations relating to
      the debt securities by deposit of funds or United States government
      obligations;
 
    - whether the debt securities are to trade in book-entry form and the terms
      and any conditions for exchanging the global security in whole or in part
      for paper certificates;
 
    - the date of any global security if other than the original issuance of the
      first debt security to be issued;
 
    - any material provisions of the applicable indenture described in this
      prospectus that do not apply to the debt securities; and
 
    - any other specific terms of the debt securities (SECTION 2.02).
 
    The terms on which a series of debt securities may be convertible into or
exchangeable for common stock or other securities of Citigroup will be set forth
in the prospectus supplement relating to such series. Such terms will include
provisions as to whether conversion or exchange is mandatory, at the option of
the holder or at the option of Citigroup. The terms may include provisions
pursuant to which the number of shares of common stock or other securities of
Citigroup to be received by the holders of such series of debt securities may be
adjusted.
 
    The debt securities will be issued only in registered form. As currently
anticipated, debt securities of a series will trade in book-entry form, and
global notes will be issued in physical (paper) form, as described below under
"Book-Entry Procedures and Settlement." Unless otherwise provided in the
accompanying prospectus supplement, debt securities denominated in United States
dollars will be issued only in denominations of $1,000 and whole multiples of
$1,000 (SECTION 2.01). The prospectus supplement relating to offered securities
denominated in a foreign or composite currency will specify the denomination of
the offered securities.
 
    The debt securities may be presented for exchange, and debt securities other
than a global security may be presented for registration of transfer, at the
principal corporate trust office of the relevant Trustee in New York City.
Holders will not have to pay any service charge for any registration of transfer
or exchange of debt securities, but Citigroup may require payment of a sum
sufficient to cover any tax or other governmental charge payable in connection
with such registration of transfer. (SECTION 2.05)
 
                                       12
<PAGE>
PAYMENT AND PAYING AGENTS
 
    Distributions on the debt securities other than those represented by global
notes will be made in the designated currency against surrender of the debt
securities at the principal corporate trust office of the relevant trustee in
New York City. Payment will be made to the registered holder at the close of
business on the record date for such payment. Interest payments will be made at
the principal corporate trust office of the relevant trustee in New York City,
or by a check mailed to the holder at his registered address (SECTIONS 2.01 AND
5.02). Payments in any other manner will be specified in the prospectus
supplement.
 
SENIOR DEBT
 
    The senior debt securities will be issued under the senior debt indenture
and will rank on an equal basis with all other unsecured debt of Citigroup
except subordinated debt.
 
SUBORDINATED DEBT
 
    The subordinated debt securities will be issued under the subordinated debt
indenture and will rank subordinated and junior in right of payment, to the
extent set forth in the subordinated debt indenture, to all "Senior
Indebtedness" (as defined below) of Citigroup.
 
    If Citigroup defaults in the payment of any principal of, or premium, if
any, or interest on any Senior Indebtedness when it becomes due and payable
after any applicable grace period, then, unless and until the default is cured
or waived or ceases to exist, Citigroup cannot make a payment on account of or
redeem or otherwise acquire the subordinated debt securities. Nevertheless,
holders of subordinated debt securities may still receive and retain:
 
    - securities of Citigroup or any other corporation provided for by a plan of
      reorganization or readjustment that are subordinate, at least to the same
      extent that the subordinated debt securities are subordinated to Senior
      Indebtedness; and
 
    - payments made from a defeasance trust as described below.
 
    If there is any insolvency, bankruptcy, liquidation or other similar
proceeding relating to Citigroup, its creditors or its property, then all Senior
Indebtedness must be paid in full before any payment may be made to any holders
of subordinated debt securities. Holders of subordinated debt securities must
return and deliver any payments received by them, other than in a plan of
reorganization or through a defeasance trust as described below, directly to the
holders of Senior Indebtedness until all Senior Indebtedness is paid in full.
(SUBORDINATED DEBT INDENTURE, SECTION 14.01).
 
    "Senior Indebtedness" means:
 
    (1) the principal, premium, if any, and interest in respect of (A)
       indebtedness of Citigroup for money borrowed and (B) indebtedness
       evidenced by securities, notes, debentures, bonds or other similar
       instruments issued by Citigroup, including the senior debt securities;
 
    (2) all capital lease obligations of Citigroup;
 
    (3) all obligations of Citigroup issued or assumed as the deferred purchase
       price of property, all conditional sale obligations of Citigroup and all
       obligations of Citigroup under any conditional sale or title retention
       agreement, but excluding trade accounts payable in the ordinary course of
       business;
 
    (4) all obligations of Citigroup in respect of any letters of credit,
       bankers acceptance, security purchase facilities and similar credit
       transactions;
 
    (5) all obligations of Citigroup in respect of interest rate swap, cap or
       other agreements, interest rate future or options contracts, currency
       swap agreements, currency future or option contracts and other similar
       agreements;
 
                                       13
<PAGE>
    (6) all obligations of the type referred to in clauses (1) through (5) of
       other persons for the payment of which Citigroup is responsible or liable
       as obligor, guarantor or otherwise; and
 
    (7) all obligations of the type referred to in clauses (1) through (6) of
       other persons secured by any lien on any property or asset of Citigroup
       whether or not such obligation is assumed by Citigroup;
 
    but Senior Indebtedness does not include:
 
       (a) subordinated debt securities;
 
       (b) any indebtedness that by its terms is subordinated to, or ranks on an
           equal basis with, subordinated debt securities; and
 
       (c) any indebtedness between or among Citigroup and its affiliates,
           including (1) any Junior Subordinated Debt, (2) any Capital
           Securities Guarantees and (3) all other debt securities and
           guarantees in respect of those debt securities issued to any other
           trust, or a trustee of such trust, partnership or other entity
           affiliated with Citigroup which is a financing vehicle of Citigroup
           in connection with the issuance by such financing vehicle of capital
           securities or other securities guaranteed by Citigroup pursuant to an
           instrument that ranks on an equal basis with, or junior to, the
           Capital Securities Guarantees.
 
    "Junior Subordinated Debt" means Citigroup's:
 
    - 7% Junior Subordinated Deferrable Interest Debentures due November 15,
      2028;
 
    - 6.850% Junior Subordinated Deferrable Interest Debentures due January 22,
      2038;
 
    - 7 5/8% Junior Subordinated Deferrable Interest Debentures due December 1,
      2036;
 
    - 7 3/4% Junior Subordinated Deferrable Interest Debentures due December 1,
      2036;
 
    - 8% Deferrable Interest Debentures due September 30, 2036;
 
    - other notes or other obligations which may be issued under the indenture,
      dated as of October 7, 1996, between Citigroup and The Chase Manhattan
      Bank, as trustee; and
 
    - indebtedness that is by its terms subordinated to, or ranks on an equal
      basis with, the Junior Subordinated Debt.
 
    "Capital Securities Guarantees" means the guarantees issued by Citigroup in
connection with:
 
    - the 7% Trust Preferred Securities of Citigroup Capital V;
 
    - the 6.850% Trust Preferred Securities of Citigroup Capital IV;
 
    - the 7 5/8% Trust Preferred Securities of Citigroup Capital III;
 
    - the 7 3/4% Trust Preferred Securities of Citigroup Capital II;
 
    - the 8% Trust Preferred Securities of Citigroup Capital I; and
 
    - any existing or future preferred or preference stock, including capital
      securities, that is by its terms subordinated to, or ranks on an equal
      basis with, the Junior Subordinated Debt.
 
COVENANTS
 
    LIMITATIONS ON LIENS.  The senior debt indenture provides that Citigroup
will not, and will not permit any Subsidiary to, incur, issue, assume or
guarantee any indebtedness for money borrowed if such indebtedness is secured by
a pledge of, lien on, or security interest in any shares of Voting Stock of any
Significant Subsidiary, without providing that each series of senior debt
securities and, at Citigroup's option, any other senior indebtedness ranking
equally and ratably with such indebtedness. This limitation shall not apply to
indebtedness secured by a pledge of, lien on or security interest in
 
                                       14
<PAGE>
any shares of Voting Stock of any corporation at the time it becomes a
Significant Subsidiary (SENIOR DEBT INDENTURE, SECTION 5.04). The subordinated
debt indenture does not contain a similar provision.
 
    "Significant Subsidiary" means a Subsidiary, including its Subsidiaries,
that meets any of the following conditions:
 
    - Citigroup's and its other Subsidiaries' investments in and advances to the
      Subsidiary exceed 10 percent of the total assets of Citigroup and its
      Subsidiaries consolidated as of the end of the most recently completed
      fiscal year;
 
    - Citigroup's and its other Subsidiaries' proportionate share of the total
      assets of the Subsidiary after intercompany eliminations exceeds 10
      percent of the total assets of Citigroup and its Subsidiaries consolidated
      as of the end of the most recently completed fiscal year; or
 
    - Citigroup's and its other Subsidiaries' equity in the income from
      continuing operations before income taxes, extraordinary items and
      cumulative effect of a change in accounting principles of the Subsidiary
      exceeds 10 percent of such income of Citigroup and its Subsidiaries
      consolidated for the most recently completed fiscal year.
 
    "Subsidiary" means any corporation of which securities entitled to elect at
least a majority of the corporation's directors shall at the time be owned,
directly or indirectly, by Citigroup, and/or one or more Subsidiaries.
 
    "Voting Stock" means capital stock the holders of which have general voting
power under ordinary circumstances to elect at least a majority of the board of
directors of a corporation, except capital stock that carries only the right to
vote conditioned on the happening of an event regardless of whether such event
shall have happened (SENIOR DEBT INDENTURE, SECTIONS 1.02 AND 5.04).
 
    LIMITATIONS ON MERGERS AND SALES OF ASSETS.  The indentures provide that
Citigroup will not merge or consolidate with another corporation or sell other
than for cash or lease all or substantially all its assets to another
corporation, or purchase all or substantially all the assets of another
corporation unless:
 
    - either (1) Citigroup is the continuing corporation, or (2) the successor
      corporation, if other than Citigroup, expressly assumes by supplemental
      indenture the obligations evidenced by the securities issued pursuant to
      the indenture and
 
    - immediately after the transaction, there would not be any default in the
      performance of any covenant or condition of the indenture (SENIOR DEBT
      INDENTURE, SECTIONS 5.05 AND 14.01; SUBORDINATED DEBT INDENTURE, SECTIONS
      5.04 AND 15.01).
 
    Other than the restrictions described above, the indentures do not contain
any covenants or provisions that would protect holders of the debt securities in
the event of a highly leveraged transaction.
 
MODIFICATION OF THE INDENTURES
 
    Under the indentures, Citigroup and the relevant trustee can enter into
supplemental indentures to establish the form and terms of any series of debt
securities without obtaining the consent of any holder of debt securities.
 
    Citigroup and the trustee may, with the consent of the holders of at least
66 2/3% in aggregate principal amount of the senior debt securities of a series
or at least a majority in aggregate principal amount of the subordinated debt
securities, modify the applicable indenture or the rights of the holders of the
securities of such series to be affected.
 
    No such modification may, without the consent of the holder of each security
so affected:
 
    - extend the fixed maturity of any such securities,
 
    - reduce the rate or extend the time of payment of interest on such
      securities,
 
                                       15
<PAGE>
    - reduce the principal amount of such securities or the premium, if any, on
      such securities,
 
    - reduce the amount of the principal of any securities issued originally at
      a discount,
 
    - change the currency in which any such securities are payable, or
 
    - impair the right to sue for the enforcement of any such payment on or
      after the maturity of such securities.
 
    In addition, no such modification may:
 
    - reduce the percentage of securities referred to above whose holders need
      to consent to the modification without the consent of such holders; or
 
    - change, without the written consent of the trustee, the rights, duties or
      immunities of the trustee (SECTIONS 13.01 AND 13.02).
 
    In addition, the subordinated debt indenture may not be amended without the
consent of each holder of subordinated debt securities affected thereby to
modify the subordination of the subordinated debt securities issued under that
indenture in a manner adverse to the holders of the subordinated debt securities
(SUBORDINATED DEBT INDENTURE, SECTION 13.02).
 
DEFAULTS
 
    Each indenture provides that events of default regarding any series of debt
securities will be:
 
    - failure to pay required interest on any debt security of such series for
      30 days;
 
    - failure to pay principal, other than a scheduled installment payment, or
      premium, if any, on any debt security of such series when due;
 
    - failure to make any required scheduled installment payment for 30 days on
      debt securities of such series;
 
    - failure to perform for 90 days after notice any other covenant in the
      relevant indenture other than a covenant included in the relevant
      indenture solely for the benefit of a series of debt securities other than
      such series; and
 
    - certain events of bankruptcy or insolvency, whether voluntary or not
      (SECTION 6.01).
 
    If an event of default regarding debt securities of any series issued under
the indentures should occur and be continuing, either the trustee or the holders
of 25% in the principal amount of outstanding debt securities of such series may
declare each debt security of that series due and payable (SECTION 6.02).
Citigroup is required to file annually with the trustee a statement of an
officer as to the fulfillment by Citigroup of its obligations under the
indenture during the preceding year (SENIOR DEBT INDENTURE, SECTION 5.06;
SUBORDINATED DEBT INDENTURE, SECTION 5.05).
 
    No event of default regarding one series of debt securities issued under an
indenture is necessarily an event of default regarding any other series of debt
securities (SECTION 6.02).
 
    Holders of a majority in principal amount of the outstanding debt securities
of any series will be entitled to control certain actions of the trustee under
the indentures and to waive past defaults regarding such series (SECTIONS 6.02
AND 6.06). The trustee generally will not be requested, ordered or directed by
any of the holders of debt securities, unless one or more of such holders shall
have offered to the trustee reasonable security or indemnity (SECTION 10.01).
 
    If an event of default occurs and is continuing regarding a series of debt
securities, the trustee may use any sums that it holds under the relevant
indenture for its own reasonable compensation and expenses incurred prior to
paying the holders of debt securities of such series (SECTION 6.05).
 
    Before any holder of any series of debt securities may institute action for
any remedy, except payment on such holder's debt security when due, the holders
of not less than 25% in principal amount
 
                                       16
<PAGE>
of the debt securities of that series outstanding must request the trustee to
take action. Holders must also offer and give the satisfactory security and
indemnity against liabilities incurred by the trustee for taking such action
(SECTION 6.07).
 
DEFEASANCE
 
    SENIOR DEBT INDENTURE.  If so specified when the senior debt securities of a
particular series is created, after Citigroup has deposited with the trustee,
cash or government securities, in trust for the benefit of the holders
sufficient to pay the principal of, premium, if any, and interest on the senior
debt securities of such series when due, then Citigroup, at its option:
 
    - will be deemed to have paid and satisfied its obligations on all
      outstanding senior debt securities of such series, which is known as
      "defeasance and discharge;" or
 
    - will cease to be under any obligation, other than to pay when due the
      principal of, premium, if any, and interest on such senior debt
      securities, relating to the senior debt securities of such series, which
      is known as "covenant defeasance."
 
    In the case of covenant defeasance, Citigroup must also deliver to the
trustee an opinion of counsel to the effect that the holders of the senior debt
securities of such series will have no federal income tax consequences as a
result of such deposit.
 
    When there is a defeasance and discharge, (1) the senior debt indenture will
no longer govern the senior debt securities of such series, (2) Citigroup will
no longer be liable for payment and (3) the holders of such senior debt
securities will be entitled only to the deposited funds. When there is a
covenant defeasance, however, Citigroup will continue to be obligated to make
payments when due if the deposited funds are not sufficient.
 
    The obligations and rights under the senior debt indenture regarding
compensation, reimbursement and indemnification of the trustee, optional
redemption, mandatory and optional scheduled installment payments, if any,
registration of transfer and exchange of the senior debt securities of such
series, replacement of mutilated, destroyed, lost or stolen senior debt
securities and certain other administrative provisions will continue even if
Citigroup exercises its defeasance and discharge or covenant defeasance options
(SENIOR DEBT INDENTURE, SECTIONS 11.03 AND 11.04).
 
    Under current federal income tax law, defeasance and discharge would be
treated as a taxable exchange of the senior debt securities for an interest in
the trust. As a consequence, each holder of the senior debt securities would
recognize gain or loss equal to the difference between the value of the holder's
interest in the trust and holder's tax basis for the senior debt securities
deemed exchanged. Each holder would then be required to include in income his
share of any income, gain and loss recognized by the trust. Even though federal
income tax on the deemed exchange would be imposed on a holder, the holder would
not receive any cash until the maturity or an earlier redemption of the senior
debt securities, except for any current interest payments.
 
    Under current federal income tax law, a covenant defeasance would not be
treated as a taxable exchange of senior debt securities. Prospective investors
are urged to consult their own tax advisors as to the specific consequences of a
defeasance and discharge, including the applicability and effect of tax laws
other than the federal income tax law.
 
    SUBORDINATED DEBT INDENTURE.  The defeasance and discharge and covenant
defeasance provisions contained in the subordinated debt indenture are
substantially the same as those described above for the senior debt indenture
(SUBORDINATED DEBT INDENTURE, SECTIONS 11.01, 11.02, 11.03, 11.04 AND 11.05).
 
    Under the subordinated debt indenture, Citigroup must also deliver to the
trustee an opinion of counsel to the effect that the holders of the subordinated
debt securities will not recognize income, gain or loss for federal income tax
purposes as a result of such deposit and defeasance and discharge or covenant
defeasance and that federal income tax would be imposed on the holders in the
same
 
                                       17
<PAGE>
manner as if such defeasance and discharge had not occurred. In the case of a
defeasance and discharge, such opinion must be based upon a ruling or
administrative pronouncement of the IRS.
 
CONCERNING THE TRUSTEES
 
    Citigroup has had and may continue to have banking relationships with the
trustees in the ordinary course of business.
 
                                       18
<PAGE>
                         DESCRIPTION OF INDEX WARRANTS
 
    The following briefly summarizes the material terms and provisions of the
index warrants, other than pricing and related terms disclosed in the
accompanying prospectus supplement. You should read the particular terms of the
index warrants that are offered by Citigroup, which will be described in more
detail in a prospectus supplement. The prospectus supplement will also state
whether any of the general provisions summarized below do not apply to the index
warrants being offered.
 
    Index warrants may be issued independently or together with debt securities
and may be attached to or separate from any such offered securities. Each series
of index warrants will be issued under a separate index warrant agreement to be
entered into between Citigroup and a bank or trust company, as index warrant
agent. A single bank or trust company may act as index warrant agent for more
than one series of index warrants. The index warrant agent will act solely as
the agent of Citigroup under the applicable index warrant agreement and will not
assume any obligation or relationship of agency or trust for or with any owners
of such index warrants. A copy of the form of index warrant agreement, including
the form of certificate or global certificate that will represent the index
warrant certificate, will be filed as an exhibit to a document incorporated by
reference in the registration statement of which this prospectus forms a part.
You should read the more detailed provisions of the index warrant agreement and
the index warrant certificate or index warrant global certificate for provisions
that may be important to you.
 
GENERAL
 
    The index warrant agreement does not limit the number of index warrants that
may be issued. Citigroup will have the right to "reopen" a previous series of
index warrants by issuing additional index warrants of such series.
 
    Each index warrant will entitle the warrant holder to receive from
Citigroup, upon exercise, cash or securities. The amount in cash or number of
securities will be determined by referring to an index calculated on the basis
of prices, yields, levels or other specified objective measures in respect of:
 
    - specified securities or securities indices;
 
    - specified foreign currencies or currency indices;
 
    - a combination thereof; or
 
    - changes in such measure or differences between two or more such measures.
 
    The prospectus supplement for a series of index warrants will describe the
formula or methodology to be applied to the relevant index or indices to
determine the amount payable or distributable on the index warrants.
 
    If so specified in the prospectus supplement, the index warrants will
entitle the warrant holder to receive from Citigroup a minimum or maximum amount
upon automatic exercise at expiration or the happening of any other event
described in the prospectus supplement.
 
    The index warrants will be deemed to be automatically exercised upon
expiration. Upon such automatic exercise, warrant holders will be entitled to
receive the cash amount or number of securities due, if any, on such exercise of
the index warrants.
 
    You should read the prospectus supplement applicable to any series of index
warrants for any circumstances in which the payment or distribution or the
determination of the payment or distribution on the index warrants may be
postponed or exercised early or cancelled. The amount due after any such delay
or postponement, or early exercise or cancellation, will be described in the
applicable prospectus supplement.
 
                                       19
<PAGE>
    Unless otherwise specified in the applicable prospectus supplement,
Citigroup will not purchase or take delivery of or sell or deliver any
securities or currencies, including the underlying assets, other than the
payment of any cash or distribution of any securities due on the index warrants,
from or to warrant holders pursuant to the index warrants.
 
    The applicable prospectus supplement relating to any series of index
warrants will describe the following:
 
    - the aggregate number of such index warrants;
 
    - the offering price of such index warrants;
 
    - the measure or measures by which payment or distribution on such index
      warrants will be determined;
 
    - certain information regarding the underlying securities, foreign
      currencies or indices;
 
    - the amount of cash or number of securities due, or the means by which the
      amount of cash or number of securities due may be calculated, on exercise
      of the index warrants, including automatic exercise, or upon cancellation;
 
    - the date on which the index warrants may first be exercised and the date
      on which they expire;
 
    - any minimum number of index warrants exercisable at any one time;
 
    - any maximum number of index warrants that may, at Citigroup's election, be
      exercised by all warrant holders or by any person or entity on any day;
 
    - any provisions permitting a warrant holder to condition an exercise of
      index warrants;
 
    - the method by which the index warrants may be exercised;
 
    - the currency in which the index warrants will be denominated and in which
      payments on the index warrants will be made or the securities that may be
      distributed in respect of the index warrants;
 
    - the method of making any foreign currency translation applicable to
      payments or distributions on the index warrants;
 
    - the method of providing for a substitute index or indices or otherwise
      determining the amount payable in connection with the exercise of index
      warrants if an index changes or is no longer available;
 
    - the time or times at which amounts will be payable or distributable in
      respect of such index warrants following exercise or automatic exercise;
 
    - any national securities exchange on, or self-regulatory organization with
      which, such index warrants will be listed;
 
    - any provisions for issuing such index warrants in certificated form;
 
    - if such index warrants are not issued in book-entry form, the place or
      places at and the procedures by which payments or distributions on the
      index warrants will be made; and
 
    - any other terms of such index warrants.
 
    Prospective purchasers of index warrants should be aware of special United
States federal income tax considerations applicable to instruments such as the
index warrants. The prospectus supplement relating to each series of index
warrants will describe these tax considerations. The summary of United States
federal income tax considerations contained in the prospectus supplement will be
presented for
 
                                       20
<PAGE>
informational purposes only, however, and will not be intended as legal or tax
advice to prospective purchasers. You are urged to consult your own tax advisors
before purchasing any index warrants.
 
LISTING
 
    Unless otherwise indicated in the prospectus supplement, the index warrants
will be listed on a national securities exchange or with a self-regulatory
organization, in each case as specified in the prospectus supplement. It is
expected that such organization will stop trading a series of index warrants as
of the close of business on the related expiration date of such index warrants.
 
MODIFICATION
 
    The index warrant agreement and the terms of the related index warrants may
be amended by Citigroup and the index warrant agent, without the consent of the
holders of any index warrants, for any of the following purposes:
 
    - curing any ambiguity or curing, correcting or supplementing any defective
      or inconsistent provision;
 
    - maintaining the listing of such index warrants on any national securities
      exchange or with any other self-regulatory organization;
 
    - registering such index warrants under the Exchange Act, permitting the
      issuance of individual index warrant certificates to warrant holders,
      reflecting the issuance by Citigroup of additional index warrants of the
      same series or reflecting the appointment of a successor depository; or
 
    - for any other purpose which Citigroup may deem necessary or desirable and
      which will not materially and adversely affect the interests of the
      warrant holders.
 
    Citigroup and the index warrant agent also may modify or amend the index
warrant agreement and the terms of the related index warrants, with the consent
of the holders of not less than a majority of the then outstanding warrants
affected by such modification or amendment, for any purpose. However, no such
modification or amendment may be made without the consent of each holder
affected thereby if such modification or amendment:
 
    - changes the amount to be paid to the warrant holder or the manner in which
      that amount is to be determined;
 
    - shortens the period of time during which the index warrants may be
      exercised;
 
    - otherwise materially and adversely affects the exercise rights of the
      holders of the index warrants; or
 
    - reduces the percentage of the number of outstanding index warrants the
      consent of whose holders is required for modification or amendment of the
      index warrant agreement or the terms of the related index warrants.
 
MERGER, CONSOLIDATION, SALE OR OTHER DISPOSITION
 
    If at any time there is a merger or consolidation involving Citigroup or a
sale, transfer, conveyance, other than lease, or other disposition of all or
substantially all of the assets of Citigroup, then the assuming corporation will
succeed to the obligations of Citigroup under the index warrant agreement and
the related index warrants. Citigroup will then be relieved of any further
obligation under the index warrant agreement and index warrants and may then be
dissolved, wound up or liquidated.
 
ENFORCEABILITY OF RIGHTS BY WARRANT HOLDERS
 
    Any warrant holder may, without the consent of the index warrant agent or
any other warrant holder, enforce by appropriate legal action on his own behalf
his right to exercise, and to receive payment for, his index warrants.
 
                                       21
<PAGE>
                          DESCRIPTION OF CAPITAL STOCK
 
GENERAL
 
    As of the date of this prospectus, Citigroup's authorized capital stock
consists of 6 billion shares of common stock and 30 million shares of preferred
stock. The following briefly summarizes the material terms of Citigroup's common
stock and outstanding preferred stock. You should read the more detailed
provisions of Citigroup's certificate of incorporation and the certificate of
designation relating to any series of preferred stock for provisions that may be
important to you.
 
COMMON STOCK
 
    As of December 31, 1998, Citigroup had outstanding approximately 2.3 billion
shares of its common stock. Each holder of common stock is entitled to one vote
per share for the election of directors and for all other matters to be voted on
by Citigroup's stockholders. Except as otherwise provided by law, the holders of
shares of common stock vote as one class together with holders of the shares of
the outstanding Series I Preferred Stock and Series K Preferred Stock, which are
described below. Holders of common stock may not cumulate their votes in the
election of directors, and are entitled to share equally in the dividends that
may be declared by the board of directors, but only after payment of dividends
required to be paid on outstanding shares of preferred stock.
 
    Upon voluntary or involuntary liquidation, dissolution or winding up of
Citigroup, the holders of the common stock share ratably in the assets remaining
after payments to creditors and provision for the preference of any preferred
stock. There are no preemptive or other subscription rights, conversion rights
or redemption or scheduled installment payment provisions relating to shares of
common stock. All of the outstanding shares of common stock are fully paid and
nonassessable. The transfer agent and registrar for the common stock is
Citibank, N.A. The common stock is listed on The New York Stock Exchange, Inc.
and The Pacific Exchange, Inc.
 
PREFERRED STOCK
 
    The general terms of Citigroup's preferred stock are described below under
"Description of Preferred Stock."
 
    As of the date of this prospectus, Citigroup had outstanding the following
series of preferred stock with the following terms:
 
<TABLE>
<CAPTION>
                                       NUMBER OF                       REDEMPTION         DATE NEXT
                                        SHARES       DIVIDENDS PER     PRICE PER         REDEEMABLE           GENERAL
TITLE OF SERIES                       OUTSTANDING        YEAR          SHARE ($)        BY CITIGROUP       VOTING RIGHTS
- ------------------------------------  -----------  -----------------  ------------  ---------------------  -------------
<S>                                   <C>          <C>                <C>           <C>                    <C>
6.365% Cumulative Preferred Stock,
  Series F..........................   1,600,000        6.365%                250       June 16, 2007           No
6.213% Cumulative Preferred Stock,
  Series G..........................     800,000        6.213%                250       July 11, 2007           No
6.231% Cumulative Preferred Stock,
  Series H..........................     800,000        6.231%                250     September 8, 2007         No
Series I Cumulative Convertible
  Preferred Stock...................     140,000(1)      $90.00             1,000    October 31, 1999(7)      Yes(10)
8.40% Cumulative Preferred Stock,
  Series K..........................     500,000        $42.00                500      March 31, 2001        Yes(10)
9.50% Cumulative Preferred Stock,
  Series L..........................          -- (2)       9.50%              500       June 30, 2001        Yes(10)
5.864% Cumulative Preferred Stock,
  Series M..........................     800,000        5.864%                250      October 8, 2007         No
Graduated Rate Cumulative Preferred
  Stock, Series O...................     625,000   Variable Rate(3)           100    August 15, 1999(8)        No
Adjustable Rate Cumulative Preferred
  Stock, Series Q...................     700,000   Variable Rate(4)           250       May 31, 1999           No
</TABLE>
 
                                       22
<PAGE>
<TABLE>
<CAPTION>
                                       NUMBER OF                       REDEMPTION         DATE NEXT
                                        SHARES       DIVIDENDS PER     PRICE PER         REDEEMABLE           GENERAL
TITLE OF SERIES                       OUTSTANDING        YEAR          SHARE ($)        BY CITIGROUP       VOTING RIGHTS
- ------------------------------------  -----------  -----------------  ------------  ---------------------  -------------
<S>                                   <C>          <C>                <C>           <C>                    <C>
Adjustable Rate Cumulative Preferred
  Stock, Series R...................     400,000   Variable Rate(4)           250      August 31, 1999          No
8.30% Noncumulative Preferred Stock,
  Series S..........................     500,000         8.30%                250     November 15, 1999         No
8 1/2% Noncumulative Preferred
  Stock, Series T...................     600,000         8.50%                250     February 15, 2000         No
7 3/4% Cumulative Preferred Stock,
  Series U..........................     500,000         7.75%                250       May 15, 2000            No
Fixed Adjustable Rate Cumulative
  Preferred Stock, Series V.........     250,000   Variable Rate(5)           500   February 15, 2006(9)        No
Cumulative Adjustable Rate Preferred
  Stock, Series Y...................       2,262   Variable Rate(6)       100,000    On any payment date        No
5.321% Cumulative Preferred Stock,
  Series YY.........................         987        5.321%          1,000,000     December 22, 2018         No
</TABLE>
 
    Where the above table indicates that the holders of the preferred stock have
no general voting rights, this means that they do not vote on matters submitted
to a vote of the common stockholders. However, the holders of this preferred
stock do have other special voting rights (1) that are required by law, (2) that
apply if there is a default in paying dividends for the equivalent of six
calendar quarters, in some cases whether or not consecutive, and (3) when
Citigroup wants to create any class of stock having a preference as to dividends
or distributions of assets over such series or alter or change the provisions of
the certificate of incorporation so as to adversely affect the powers,
preferences or rights of the holders of such series. These special voting rights
apply to all series of preferred stock listed above.
 
- ------------------------------
 
(1) Each share of Series I Preferred Stock is convertible at the holder's option
    into 44.60526 shares of common stock, subject to anti-dilution adjustment.
 
(2) The Series L Preferred Stock will be issuable upon the settlement of
    purchase contracts issued as a component of the 9 1/2% Trust Preferred Stock
    Units of SI Financing Trust I, a subsidiary of Salomon Smith Barney Holdings
    Inc. Holders must settle the purchase contracts on June 30, 2021; however,
    SSBH has the option to accelerate settlement to June 30, 2001.
 
(3) For all quarterly dividend periods ending on or prior to August 15, 1999,
    the annual dividend rate is the Five-Year Treasury Rate (as defined in the
    Series O Preferred Stock Certificate of Designation) plus 1.50%. Between
    August 15, 1999 and August 15, 2004, the annual dividend rate will be the
    Five-Year Treasury Rate plus 2.25%. Prior to August 15, 2004, the above
    rates may not be less than 7.00% nor greater than 14.00% per year, and after
    August 15, 2004, the rates may not be less than 8.00% nor greater than
    16.00% per year. The dividend rate will be increased in the event of
    specified changes in the Internal Revenue Code that would decrease the
    dividends received deduction applicable to corporate stockholders (a "Change
    in Tax Law").
 
(4) For each dividend period the dividend rate will be equal to 84% of the
    Effective Rate, but not less than 4.50% per year or more than 10.50% per
    year. The "Effective Rate" for any dividend period will be equal to the
    highest of the Treasury Bill Rate, the Ten Year Constant Maturity Rate and
    the Thirty Year Constant Maturity Rate, each as defined in the relevant part
    of the Certificate of Incorporation.
 
(5) The Series V Preferred Stock provides for a cumulative dividend at a rate
    that changes over time. For each dividend period up to but not including
    February 15, 2006, the dividend rate will be 5.86% per year. For each
    dividend period beginning on or after February 15, 2006, the dividend rate
    will be equal to 0.50% plus the Effective Rate, but not less than 6.00% or
    more than 12.00%. The dividend rate will be increased in the event of a
    Change in Tax Law.
 
(6) The holders of the Series Y Preferred Stock are entitled to a cumulative
    quarterly dividend at an annual rate equal to the greater of (a) the Short
    Term Rate and (b) 4.85%. The "Short Term Rate" generally will be equal to
    either 85% or 78% of the Money Market Yield, as defined in the certificate
    of incorporation, of the 90-day rate for commercial paper multiplied by the
    stock's $100,000 per share liquidation value.
 
(7) Redemption is mandatory.
 
(8) If not redeemed on August 15, 1999, also redeemable at any time after August
    15, 2004 or following a Change in Tax Law.
 
(9) Prior to February 15, 2006, in the event of a Change in Tax Law, Citigroup
    at its option may redeem all, but not less than all, of the Series V
    Preferred Stock at a price declining over time from $525 per share to $500
    per share.
 
                                       23
<PAGE>
(10) Holders of shares of Series I Preferred Stock are entitled to 44.60526
    votes per share, and holders of shares of Series K and L Preferred Stock are
    entitled to three votes per share, when voting as a class with the common
    stock, subject to anti-dilution adjustment. The shares of Series I Preferred
    Stock are entitled to vote together as a class with the shares of common
    stock and any other shares of capital stock also then entitled to vote as a
    class with the common stock on all matters submitted to a vote of
    stockholders of Citigroup. In addition, holders of Series I and K Preferred
    Stock together with all other series of preferred stock, voting as one
    class, must give their approval by a two-thirds vote of shares of preferred
    stock then outstanding to some types of liquidation, merger and asset sale
    transactions. Finally, without obtaining the approval of a majority of the
    outstanding shares of Series I and K Preferred Stock voting separately as a
    class, Citigroup may not amend the certificate of incorporation to increase
    the authorized amount of preferred stock or to authorize any other stock
    ranking on a parity with the preferred stock either as to payment of
    dividends or upon liquidation.
 
IMPORTANT PROVISIONS OF CITIGROUP'S CERTIFICATE OF INCORPORATION AND BY-LAWS
 
    BUSINESS COMBINATIONS.  The certificate of incorporation generally requires
the affirmative vote of at least 66 2/3% of the votes entitled to be cast by the
holders of the then outstanding shares of voting stock, voting together as a
single class, to approve any merger or other business combination between
Citigroup and any interested stockholder, unless (1) the transaction has been
approved by a majority of the continuing directors of Citigroup, or (2) minimum
price, form of consideration and procedural requirements are satisfied. An
"interested stockholder" as defined in the certificate of incorporation
generally means a person who owns at least 25% of the voting stock of Citigroup.
 
    AMENDMENTS TO CERTIFICATE OF INCORPORATION AND BY-LAWS.  The affirmative
vote of the holders of at least 75% of the voting power of the shares entitled
to vote for the election of directors is required to amend the provisions of the
certificate of incorporation relating to the issuance of preferred stock or
common stock. Amendments of provisions of the certificate of incorporation
relating to business combinations generally require a vote of the holders of
66 2/3% of the then outstanding shares of voting stock, unless 75% of the board
of directors recommend such amendment and the directors comprising such 75%
would qualify as continuing directors. The board of directors, at any meeting,
may alter or amend the by-laws upon the affirmative vote of at least 66 2/3% of
the entire board of directors. A "continuing director" as defined in the
certificate of incorporation generally means a director who is not related to an
interested stockholder who held that position before an interested stockholder
became an interested stockholder.
 
    VACANCIES.  Vacancies on the board of directors resulting from an increase
in the number of directors may be filled by a majority of the board of directors
then in office, so long as a quorum is present. Any other vacancies on the board
of directors may be filled by a majority of the directors then in office, even
if less than a quorum. Any director elected to fill a vacancy that did not
result from increasing the size of the board of directors shall hold office for
a term coinciding with the predecessor director's remaining term.
 
                                       24
<PAGE>
                         DESCRIPTION OF PREFERRED STOCK
 
    The following briefly summarizes the material terms of Citigroup's preferred
stock, other than pricing and related terms disclosed in the accompanying
prospectus supplement. You should read the particular terms of any series of
preferred stock offered by Citigroup which will be described in more detail in
any prospectus supplement relating to such series, together with the more
detailed provisions of Citigroup's restated certificate of incorporation and the
certificate of designation relating to each particular series of preferred stock
for provisions that may be important to you. The certificate of incorporation
and a certificate of designation relating to the Series YY Preferred Stock are
filed as exhibits to the registration statement of which this prospectus forms a
part. The certificate of designation relating to the particular series of
preferred stock offered by the accompanying prospectus supplement and this
prospectus will be filed as an exhibit to a document incorporated by reference
in the registration statement. The prospectus supplement will also state whether
any of the terms summarized below do not apply to the series of preferred stock
being offered. For a description of Citigroup's outstanding preferred stock, see
"Description of Capital Stock."
 
GENERAL
 
    Under Citigroup's certificate of incorporation, the board of directors of
Citigroup is authorized to issue shares of preferred stock in one or more
series, and to establish from time to time a series of preferred stock with the
following terms specified:
 
    - the number of shares to be included in the series;
 
    - the designation, powers, preferences and rights of the shares of the
      series; and
 
    - the qualifications, limitations or restrictions of such series, except as
      otherwise stated in the certificate of incorporation.
 
    Prior to the issuance of any series of preferred stock, the board of
directors of Citigroup will adopt resolutions creating and designating the
series as a series of preferred stock and the resolutions will be filed in a
certificate of designation as an amendment to the certificate of incorporation.
The term "board of directors of Citigroup" includes any duly authorized
committee.
 
    The rights of holders of the preferred stock offered may be adversely
affected by the rights of holders of any shares of preferred stock that may be
issued in the future. The board of directors may cause shares of preferred stock
to be issued in public or private transactions for any proper corporate purpose.
Examples of proper corporate purposes include issuances to obtain additional
financing in connection with acquisitions or otherwise, and issuances to
officers, directors and employees of Citigroup and its subsidiaries pursuant to
benefit plans or otherwise. Shares of preferred stock issued by Citigroup may
have the effect of rendering more difficult or discouraging an acquisition of
Citigroup deemed undesirable by the board of directors of Citigroup.
 
    Under existing interpretations of The Board of Governors of the Federal
Reserve System and the Office of Thrift Supervision, if the holders of the
preferred stock become entitled to vote for the election of directors because
dividends on the preferred stock are in arrears as described below, preferred
stock may then be deemed a "class of voting securities" and a holder of 25% or
more of the preferred stock or a holder of 5% or more of the preferred stock
that otherwise exercises a "controlling influence" over Citigroup may then be
regulated as a "bank holding company" in accordance with the Bank Holding
Company Act. Moreover, a holder of 25% or more of the preferred stock or a
holder of 10% or more of the preferred stock that otherwise possesses certain
"control factors" relating
 
                                       25
<PAGE>
to Citigroup may then be regulated as a "savings and loan holding company" in
accordance with the Home Owner's Loan Act of 1933. In addition, at such time:
 
    - any bank holding company or foreign bank with a U.S. presence generally
      would be required to obtain the approval of the Federal Reserve Board
      under the BHC Act to acquire or retain 5% or more of the preferred stock;
 
    - any person other than a bank holding company may be required to obtain the
      approval of the Federal Reserve Board and the OTS under the Change in Bank
      Control Act to acquire or retain 10% or more of the preferred stock; and
 
    - any savings and loan holding company generally could not retain in excess
      of 5% of the preferred stock.
 
Before exercising its option to redeem any shares of preferred stock, Citigroup
will obtain the approval of the Federal Reserve Board if then required by
applicable law.
 
    The preferred stock will be, when issued, fully paid and nonassessable.
Holders of preferred stock will not have any preemptive or subscription rights
to acquire more stock of Citigroup.
 
    The transfer agent, registrar, dividend disbursing agent and redemption
agent for shares of each series of preferred stock will be named in the
prospectus supplement relating to such series.
 
RANK
 
    Unless otherwise specified in the prospectus supplement relating to the
shares of any series of preferred stock, such shares will rank on an equal basis
with each other series of preferred stock and prior to the common stock as to
dividends and distributions of assets.
 
DIVIDENDS
 
    Holders of each series of preferred stock will be entitled to receive cash
dividends, when, as and if declared by the board of directors of Citigroup out
of funds legally available for dividends. The rates and dates of payment of
dividends will be set forth in the prospectus supplement relating to each series
of preferred stock. Dividends will be payable to holders of record of preferred
stock as they appear on the books of Citigroup or, if applicable, the records of
the depositary referred to below under "Description of Depositary Shares," on
the record dates fixed by the board of directors. Dividends on any series of
preferred stock may be cumulative or noncumulative.
 
    Citigroup may not declare, pay or set apart for payment dividends on the
preferred stock unless full dividends on any other series of preferred stock
that ranks on an equal or senior basis have been paid or sufficient funds have
been set apart for payment for
 
    - all prior dividend periods of the other series of preferred stock that pay
      dividends on a cumulative basis; or
 
    - the immediately preceding dividend period of the other series of preferred
      stock that pay dividends on a noncumulative basis.
 
    Partial dividends declared on shares of preferred stock and any other series
of preferred stock ranking on an equal basis as to dividends will be declared
pro rata. A pro rata declaration means that the ratio of dividends declared per
share to accrued dividends per share will be the same for both series of
preferred stock.
 
    Similarly, Citigroup may not declare, pay or set apart for payment non-stock
dividends or make other payments on the common stock or any other stock of
Citigroup ranking junior to the preferred stock until full dividends on the
preferred stock have been paid or set apart for payment for
 
                                       26
<PAGE>
    - all prior dividend periods if the preferred stock pays dividends on a
      cumulative basis; or
 
    - the immediately preceding dividend period if the preferred stock pays
      dividends on a noncumulative basis.
 
CONVERSION AND EXCHANGE
 
    The prospectus supplement for any series of preferred stock will state the
terms, if any, on which shares of that series are convertible into or
exchangeable for shares of Citigroup's common stock.
 
REDEMPTION
 
    If so specified in the applicable prospectus supplement, a series of
preferred stock may be redeemable at any time, in whole or in part, at the
option of Citigroup or the holder thereof and may be mandatorily redeemed.
 
    Any partial redemptions of preferred stock will be made in a way that the
board of directors decides is equitable.
 
    Unless Citigroup defaults in the payment of the redemption price, dividends
will cease to accrue after the redemption date on shares of preferred stock
called for redemption and all rights of holders of such shares will terminate
except for the right to receive the redemption price.
 
LIQUIDATION PREFERENCE
 
    Upon any voluntary or involuntary liquidation, dissolution or winding up of
Citigroup, holders of each series of preferred stock will be entitled to receive
distributions upon liquidation in the amount set forth in the prospectus
supplement relating to such series of preferred stock, plus an amount equal to
any accrued and unpaid dividends. Such distributions will be made before any
distribution is made on any securities ranking junior relating to liquidation,
including common stock.
 
    If the liquidation amounts payable relating to the preferred stock of any
series and any other securities ranking on a parity regarding liquidation rights
are not paid in full, the holders of the preferred stock of such series and such
other securities will share in any such distribution of available assets of
Citigroup on a ratable basis in proportion to the full liquidation preferences.
Holders of such series of preferred stock will not be entitled to any other
amounts from Citigroup after they have received their full liquidation
preference.
 
VOTING RIGHTS
 
    The holders of shares of preferred stock will have no voting rights, except:
 
    - as otherwise stated in the prospectus supplement;
 
    - as otherwise stated in the certificate of designation establishing such
      series; or
 
    - as required by applicable law.
 
                                       27
<PAGE>
                        DESCRIPTION OF DEPOSITARY SHARES
 
    The following briefly summarizes the material provisions of the deposit
agreement and of the depositary shares and depositary receipts, other than
pricing and related terms disclosed in the accompanying prospectus supplement.
You should read the particular terms of any depositary shares and any depositary
receipts that are offered by Citigroup and any deposit agreement relating to a
particular series of preferred stock which will be described in more detail in a
prospectus supplement. The prospectus supplement will also state whether any of
the generalized provisions summarized below do not apply to the depositary
shares or depositary receipts being offered. A copy of the form of deposit
agreement, including the form of depositary receipt, is incorporated by
reference as an exhibit in the registration statement of which this prospectus
forms a part. You should read the more detailed provisions of the deposit
agreement and the form of depositary receipt for provisions that may be
important to you.
 
GENERAL
 
    Citigroup may, at its option, elect to offer fractional shares of preferred
stock, rather than full shares of preferred stock. In such event, Citigroup will
issue receipts for depositary shares, each of which will represent a fraction of
a share of a particular series of preferred stock.
 
    The shares of any series of preferred stock represented by depositary shares
will be deposited under a deposit agreement between Citigroup and a bank or
trust company selected by Citigroup having its principal office in the United
States and having a combined capital and surplus of at least $50,000,000, as
preferred stock depositary. Each owner of a depositary share will be entitled to
all the rights and preferences of the underlying preferred stock, including
dividend, voting, redemption, conversion and liquidation rights, in proportion
to the applicable fraction of a share of preferred stock represented by such
depositary share.
 
    The depositary shares will be evidenced by depositary receipts issued
pursuant to the deposit agreement. Depositary receipts will be distributed to
those persons purchasing the fractional shares of preferred stock in accordance
with the terms of the applicable prospectus supplement.
 
DIVIDENDS AND OTHER DISTRIBUTIONS
 
    The preferred stock depositary will distribute all cash dividends or other
cash distributions received in respect of the deposited preferred stock to the
record holders of depositary shares relating to such preferred stock in
proportion to the number of such depositary shares owned by such holders.
 
    The preferred stock depositary will distribute any property received by it
other than cash to the record holders of depositary shares entitled thereto. If
the preferred stock depositary determines that it is not feasible to make such
distribution, it may, with the approval of Citigroup, sell such property and
distribute the net proceeds from such sale to such holders.
 
REDEMPTION OF PREFERRED STOCK
 
    If a series of preferred stock represented by depositary shares is to be
redeemed, the depositary shares will be redeemed from the proceeds received by
the preferred stock depositary resulting from the redemption, in whole or in
part, of such series of preferred stock. The depositary shares will be redeemed
by the preferred stock depositary at a price per depositary share equal to the
applicable fraction of the redemption price per share payable in respect of the
shares of preferred stock so redeemed.
 
    Whenever Citigroup redeems shares of preferred stock held by the preferred
stock depositary, the preferred stock depositary will redeem as of the same date
the number of depositary shares representing shares of preferred stock so
redeemed. If fewer than all the depositary shares are to be redeemed,
 
                                       28
<PAGE>
the depositary shares to be redeemed will be selected by the preferred stock
depositary by lot or ratably or by any other equitable method as the preferred
stock depositary may decide.
 
WITHDRAWAL OF PREFERRED STOCK
 
    Unless the related depositary shares have previously been called for
redemption, any holder of depositary shares may receive the number of whole
shares of the related series of preferred stock and any money or other property
represented by such depositary receipts after surrendering the depositary
receipts at the corporate trust office of the preferred stock depositary.
Holders of depositary shares making such withdrawals will be entitled to receive
whole shares of preferred stock on the basis set forth in the related prospectus
supplement for such series of preferred stock.
 
    However, holders of such whole shares of preferred stock will not be
entitled to deposit such preferred stock under the deposit agreement or to
receive depositary receipts for such preferred stock after such withdrawal. If
the depositary shares surrendered by the holder in connection with such
withdrawal exceed the number of depositary shares that represent the number of
whole shares of preferred stock to be withdrawn, the preferred stock depositary
will deliver to such holder at the same time a new depositary receipt evidencing
such excess number of depositary shares.
 
VOTING DEPOSITED PREFERRED STOCK
 
    Upon receipt of notice of any meeting at which the holders of any series of
deposited preferred stock are entitled to vote, the preferred stock depositary
will mail the information contained in such notice of meeting to the record
holders of the depositary shares relating to such series of preferred stock.
Each record holder of such depositary shares on the record date will be entitled
to instruct the preferred stock depositary to vote the amount of the preferred
stock represented by such holder's depositary shares. The preferred stock
depositary will try to vote the amount of such series of preferred stock
represented by such depositary shares in accordance with such instructions.
 
    Citigroup will agree to take all reasonable actions that the preferred stock
depositary determines as necessary to enable the preferred stock depositary to
vote as instructed. The preferred stock depositary will vote all shares of any
series of preferred stock held by it proportionately with instructions received
if it does not receive specific instructions from the holders of depositary
shares representing such series of preferred stock.
 
AMENDMENT AND TERMINATION OF THE DEPOSIT AGREEMENT
 
    The form of depositary receipt evidencing the depositary shares and any
provision of the deposit agreement may at any time be amended by agreement
between Citigroup and the preferred stock depositary. However, any amendment
that imposes additional charges or materially and adversely alters any
substantial existing right of the holders of depositary shares will not be
effective unless such amendment has been approved by the holders of at least a
majority of the affected depositary shares then outstanding. Every holder of an
outstanding depositary receipt at the time any such amendment becomes effective,
or any transferee of such holder, shall be deemed, by continuing to hold such
depositary receipt, or by reason of the acquisition thereof, to consent and
agree to such amendment and to be bound by the deposit agreement, which has been
amended thereby. The deposit agreement automatically terminates if:
 
    - all outstanding depositary shares have been redeemed;
 
    - each share of preferred stock has been converted into or exchanged for
      common stock; or
 
    - a final distribution in respect of the preferred stock has been made to
      the holders of depositary shares in connection with any liquidation,
      dissolution or winding up of Citigroup.
 
                                       29
<PAGE>
The deposit agreement may be terminated by Citigroup at any time and the
preferred stock depositary will give notice of such termination to the record
holders of all outstanding depositary receipts not less than 30 days prior to
the termination date. In such event, the preferred stock depositary will deliver
or make available for delivery to holders of depositary shares, upon surrender
of such depositary shares, the number of whole or fractional shares of the
related series of preferred stock as are represented by such depositary shares.
 
CHARGES OF PREFERRED STOCK DEPOSITARY; TAXES AND OTHER GOVERNMENTAL CHARGES
 
    No fees, charges and expenses of the preferred stock depositary or any agent
of the preferred stock depositary or of any registrar shall be payable by any
person other than Citigroup, except for any taxes and other governmental charges
and except as provided in the deposit agreement. If the preferred stock
depositary incurs fees, charges or expenses for which it is not otherwise liable
hereunder at the election of a holder of a depositary receipt or other person,
such holder or other person will be liable for such fees, charges and expenses.
 
RESIGNATION AND REMOVAL OF DEPOSITARY
 
    The preferred stock depositary may resign at any time by delivering to
Citigroup notice of its intent to do so, and Citigroup may at any time remove
the preferred stock depositary, any such resignation or removal to take effect
upon the appointment of a successor preferred stock depositary and its
acceptance of such appointment. Such successor preferred stock depositary must
be appointed within 60 days after delivery of the notice of resignation or
removal and must be a bank or trust company having its principal office in the
United States and having a combined capital and surplus of at least $50,000,000.
 
MISCELLANEOUS
 
    The preferred stock depositary will forward all reports and communications
from Citigroup which are delivered to the preferred stock depositary and which
Citigroup is required to furnish to the holders of the deposited preferred
stock.
 
    Neither the preferred stock depositary nor Citigroup will be liable if it is
prevented or delayed by law or any circumstances beyond its control in
performing its obligations under the deposit agreement. The obligations of
Citigroup and the preferred stock depositary under the deposit agreement will be
limited to performance with honest intentions of their duties thereunder and
they will not be obligated to prosecute or defend any legal proceeding in
respect of any depositary shares, depositary receipts or shares of preferred
stock unless satisfactory indemnity is furnished. Citigroup and the preferred
stock depositary may rely upon written advice of counsel or accountants, or upon
information provided by holders of depositary receipts or other persons believed
to be competent and on documents believed to be genuine.
 
                                       30
<PAGE>
                      BOOK-ENTRY PROCEDURES AND SETTLEMENT
 
    Most series of debt securities and index warrants will be book-entry
securities. Upon issuance, all book-entry securities of the same issue will be
represented by one or more fully registered global securities, without interest
coupons. Each global security will be deposited with, or on behalf of, The
Depository Trust Company, a securities depository, and will be registered in the
name of DTC or a nominee of DTC. DTC will thus be the only registered holder of
these debt securities or index warrants and will be considered the sole owner of
the securities for purposes of the indenture or index warrant agreement.
 
    Purchasers may only hold interests in the global notes or index warrants
through DTC if they are a participant in the DTC system. Purchasers may also
hold interests through a securities intermediary-- banks, brokerage houses and
other institutions that maintain securities accounts for customers--that has an
account with DTC or its nominee. DTC will maintain accounts showing the
securities holdings of its participants, and these participants will in turn
maintain accounts showing the securities holdings of their customers. Some of
these customers may themselves be securities intermediaries holding debt
securities or index warrants for their customers. Thus, each beneficial owner of
a book-entry security will hold that security indirectly through a hierarchy of
intermediaries, with DTC at the "top" and the beneficial owner's own securities
intermediary at the "bottom."
 
    The securities of each beneficial owner of a book-entry security will be
evidenced solely by entries on the books of the beneficial owner's securities
intermediary. The actual purchaser of the securities will generally not be
entitled to have the securities represented by the global securities registered
in its name and will not be considered the owner under the indenture or index
warrant agreement. In most cases, a beneficial owner will also not be able to
obtain a paper certificate evidencing the holder's ownership of securities. The
book-entry system for holding securities eliminates the need for physical
movement of certificates and is the system through which most publicly traded
common stock is held in the United States. However, the laws of some
jurisdictions require some purchasers of securities to take physical delivery of
their securities in definitive form. These laws may impair the ability to
transfer book-entry securities.
 
    A beneficial owner of book-entry securities represented by a global security
may exchange the securities for definitive (paper) securities only if:
 
    (a) DTC is unwilling or unable to continue as depositary for such global
       security and Citigroup is unable to find a qualified replacement for DTC
       within 90 days;
 
    (b) at any time DTC ceases to be a clearing agency registered under the
       Securities Exchange Act of 1934; or
 
    (c) Citigroup in its sole discretion decides to allow some or all book-entry
       securities to be exchangeable for definitive securities in registered
       form.
 
    Unless we indicate otherwise in the applicable prospectus supplement, any
global security that is exchangeable will be exchangeable in whole for
definitive securities in registered form, with the same terms and of an equal
aggregate principal amount, in denominations of $1,000 and whole multiples of
$1,000. Definitive notes or index warrants will be registered in the name or
names of the person or persons specified by DTC in a written instruction to the
registrar of the securities. DTC may base its written instruction upon
directions it receives from its participants.
 
    In this prospectus and the accompanying prospectus supplement, for
book-entry securities, references to actions taken by security holders will mean
actions taken by DTC upon instructions from its participants, and references to
payments and notices of redemption to security holders and notices of redemption
to DTC as the registered holder of the securities for distribution to
participants in accordance with DTC's procedures.
 
                                       31
<PAGE>
    DTC is a limited purpose trust company organized under the laws of the State
of New York, a member of the Federal Reserve System, a "clearing corporation"
within the meaning of the New York Uniform Commercial Code and a "clearing
agency" registered under section 17A of the Securities Exchange Act of 1934. The
rules applicable to DTC and its participants are on file with the SEC.
 
    DTC's management is aware that some computer applications, systems, and the
like for processing dates that are dependent upon calendar dates, including
dates before, on, and after January 1, 2000, may encounter "Year 2000 problems."
DTC has informed its participants and other members of the financial community
that it has developed and is implementing a program so that its systems, as they
relate to the timely payment of distributions to securityholders, book-entry
deliveries, and settlement of trades within DTC, continue to function
appropriately. This program includes a technical assessment and a remediation
plan, each of which is complete. Additionally, DTC's plan includes a testing
phase, which is expected to be completed within appropriate time frames.
 
    Citigroup will not have any responsibility or liability for any aspect of
the records relating to, or payments made on account of, beneficial ownership
interests in the book-entry securities or for maintaining, supervising or
reviewing any records relating to the beneficial ownership interests.
 
                                       32
<PAGE>
                              PLAN OF DISTRIBUTION
 
    Citigroup may offer the offered securities in one or more of the following
ways from time to time:
 
    - to or through underwriters or dealers;
 
    - by itself directly;
 
    - through agents; or
 
    - through a combination of any of these methods of sale.
 
    Any such underwriters, dealers or agents may include any broker-dealer
subsidiary of Citigroup.
 
    The prospectus supplement relating to an offering of offered securities will
set forth the terms of such offering, including:
 
    - the name or names of any underwriters, dealers or agents;
 
    - the purchase price of the offered securities and the proceeds to Citigroup
      from such sale;
 
    - any underwriting discounts and commissions or agency fees and other items
      constituting underwriters' or agents' compensation;
 
    - the initial public offering price;
 
    - any discounts or concessions to be allowed or reallowed or paid to
      dealers; and
 
    - any securities exchanges on which such offered securities may be listed.
 
Any initial public offering prices, discounts or concessions allowed or
reallowed or paid to dealers may be changed from time to time.
 
    If underwriters are used in an offering of offered securities, such offered
securities will be acquired by the underwriters for their own account and may be
resold from time to time in one or more transactions, including negotiated
transactions, at a fixed public offering price or at varying prices determined
at the time of sale. The securities may be either offered to the public through
underwriting syndicates represented by one or more managing underwriters or by
one or more underwriters without a syndicate. Unless otherwise set forth in the
prospectus supplement, the underwriters will not be obligated to purchase
offered securities unless specified conditions are satisfied, and if the
underwriters do purchase any offered securities, they will purchase all offered
securities.
 
    In connection with underwritten offerings of the offered securities and in
accordance with applicable law and industry practice, underwriters may
over-allot or effect transactions that stabilize, maintain or otherwise affect
the market price of the offered securities at levels above those that might
otherwise prevail in the open market, including by entering stabilizing bids,
effecting syndicate covering transactions or imposing penalty bids, each of
which is described below.
 
    - A stabilizing bid means the placing of any bid, or the effecting of any
      purchase, for the purpose of pegging, fixing or maintaining the price of a
      security.
 
    - A syndicate covering transaction means the placing of any bid on behalf of
      the underwriting syndicate or the effecting of any purchase to reduce a
      short position created in connection with the offering.
 
    - A penalty bid means an arrangement that permits the managing underwriter
      to reclaim a selling concession from a syndicate member in connection with
      the offering when offered securities originally sold by the syndicate
      member are purchased in syndicate covering transactions.
 
                                       33
<PAGE>
    These transactions may be effected on the NYSE, in the over-the-counter
market, or otherwise. Underwriters are not required to engage in any of these
activities, or to continue such activities if commenced.
 
    If dealers are utilized in the sale of offered securities, Citigroup will
sell such offered securities to the dealers as principals. The dealers may then
resell such offered securities to the public at varying prices to be determined
by such dealers at the time of resale. The names of the dealers and the terms of
the transaction will be set forth in the prospectus supplement relating to that
transaction.
 
    Offered securities may be sold directly by Citigroup to one or more
institutional purchasers, or through agents designated by Citigroup from time to
time, at a fixed price or prices, which may be changed, or at varying prices
determined at the time of sale. Any agent involved in the offer or sale of the
offered securities in respect of which this prospectus is delivered will be
named, and any commissions payable by Citigroup to such agent will be set forth,
in the prospectus supplement relating to that offering. Unless otherwise
indicated in such prospectus supplement, any such agent will be acting on a best
efforts basis for the period of its appointment.
 
    As one of the means of direct issuance of offered securities, Citigroup may
utilize the services of an entity through which it may conduct an electronic
"dutch auction" or similar offering of the offered securities among potential
purchasers who are eligible to participate in the auction or offering of such
offered securities, if so described in the applicable prospectus supplement.
 
    If so indicated in the applicable prospectus supplement, Citigroup will
authorize agents, underwriters or dealers to solicit offers from certain types
of institutions to purchase offered securities from Citigroup at the public
offering price set forth in such prospectus supplement pursuant to delayed
delivery contracts providing for payment and delivery on a specified date in the
future. Such contracts will be subject only to those conditions set forth in the
prospectus supplement and the prospectus supplement will set forth the
commission payable for solicitation of such contracts.
 
    The broker-dealer subsidiaries of Citigroup are members of the National
Association of Securities Dealers, Inc. and may participate in distributions of
the offered securities. Accordingly, offerings of offered securities in which
Citigroup's broker-dealer subsidiaries participate will conform with the
requirements set forth in Rule 2720 of the Conduct Rules of the NASD.
 
    This prospectus, together with any applicable prospectus supplement may also
be used by any broker-dealer subsidiary of Citigroup in connection with offers
and sales of the offered securities in market-making transactions at negotiated
prices related to prevailing market prices at the time of sale. Any of
Citigroup's broker-dealer subsidiaries, including Salomon Smith Barney Inc., may
act as principal or agent in such transactions. None of Citigroup's
broker-dealer subsidiaries have any obligation to make a market in any of the
offered securities and may discontinue any market-making activities at any time
without notice, at its sole discretion.
 
    Underwriters, dealers and agents may be entitled, under agreements with
Citigroup, to indemnification by Citigroup relating to material misstatements
and omissions. Underwriters, dealers and agents may be customers of, engage in
transactions with, or perform services for, Citigroup and affiliates of
Citigroup in the ordinary course of business.
 
    Each series of offered securities will be a new issue of securities and will
have no established trading market. Any underwriters to whom offered securities
are sold for public offering and sale may make a market in such offered
securities, but such underwriters will not be obligated to do so and may
discontinue any market making at any time without notice. The offered securities
may or may not be listed on a national securities exchange. No assurance can be
given that there will be a market for the offered securities.
 
                                       34
<PAGE>
                                 ERISA MATTERS
 
    Citigroup has subsidiaries, including insurance company subsidiaries and its
broker-dealer subsidiaries, that provide services to many employee benefit
plans. Citigroup and any direct or indirect subsidiary of Citigroup may each be
considered a "party in interest" within the meaning of the Employee Retirement
Income Security Act of 1974, and a "disqualified person" under corresponding
provisions of the Internal Revenue Code of 1986, relating to many employee
benefit plans. "Prohibited transactions" within the meaning of ERISA and the
Code may result if any offered securities are acquired by an employee benefit
plan relating to which Citigroup or any direct or indirect subsidiary of
Citigroup is a party in interest, unless such offered securities are acquired
pursuant to an applicable exemption. Any employee benefit plan or other entity
subject to which such provisions of ERISA or the Code apply proposing to acquire
the offered securities should consult with its legal counsel.
 
                                 LEGAL MATTERS
 
    Stephanie B. Mudick, Esq., General Counsel-Corporate Law and an Assistant
Secretary of Citigroup, 153 East 53(rd) Street, New York, New York 10043 and/or
Skadden, Arps, Slate, Meagher & Flom LLP, New York, New York, or counsel to be
identified in the applicable prospectus supplement, will act as legal counsel to
Citigroup. Ms. Mudick beneficially owns, or has rights to acquire under
Citigroup's employee benefit plans, an aggregate of less than 1% of Citigroup's
common stock. Dewey Ballantine LLP, New York, New York, or other counsel
identified in the applicable prospectus supplement, will act as legal counsel to
the underwriters. Dewey Ballantine LLP has from time to time acted as counsel
for Citigroup and its subsidiaries and may do so in the future. Kenneth J.
Bialkin, a partner of Skadden, Arps, Slate, Meagher & Flom LLP, is a director of
Citigroup, and he and other attorneys in such firm beneficially own an aggregate
of less than 1% of the common stock of Citigroup. A member of Dewey Ballantine
LLP participating in this matter is the beneficial owner of an aggregate of less
than 1% of Citigroup's common stock.
 
                                    EXPERTS
 
    The consolidated financial statements of Citigroup Inc. as of December 31,
1998 and 1997, and for each of the years in the three-year period ended December
31, 1998, have been audited by KPMG LLP, independent certified public
accountants, as set forth in their report on the consolidated financial
statements. The consolidated financial statements are included in Citigroup's
annual report on Form 10-K for the year ended December 31, 1998, and
incorporated by reference in this prospectus. The report of KPMG LLP also is
incorporated by reference in this prospectus. This report states that KPMG LLP
did not audit the consolidated financial statements of Salomon Inc and its
subsidiaries for the year ended December 31, 1996, appearing in Salomon Inc's
annual report on Form 10-K for the year ended December 31, 1996. The report also
states that their opinion regarding any amounts derived from the Salomon
financial statements is based on the report of Arthur Andersen LLP. The
consolidated financial statements of Citigroup referred to above are
incorporated by reference in this prospectus in reliance upon such reports and
upon the authority of said firms as experts in accounting and auditing. To the
extent that KPMG LLP audits and reports on consolidated financial statements of
Citigroup issued at future dates, and consents to the use of their report
thereon, such consolidated financial statements also will be incorporated by
reference in the registration statement in reliance upon their report and said
authority.
 
                                       35
<PAGE>
                              PRINCIPAL OFFICE OF
                                 CITIGROUP INC.
 
                              153 East 53rd Street
                            New York, New York 10043
 
                                    TRUSTEE
                           AND NEW YORK PAYING AGENT
 
                              The Bank of New York
                               101 Barclay Street
                            New York, New York 10286
 
                    LUXEMBOURG STOCK EXCHANGE LISTING AGENT
                 AND LUXEMBOURG PAYING AGENT AND TRANSFER AGENT
 
                    Banque Internationale a Luxembourg S.A.
                                69 route d'Esch
                               L-1470 Luxembourg
 
                                 LEGAL ADVISERS
 
<TABLE>
<S>                                            <C>
           TO CITIGROUP INC. AS TO                 SPECIAL UNITED STATES TAX COUNSEL TO
              UNITED STATES LAW                               CITIGROUP INC.
 
          Stephanie B. Mudick, Esq.              Skadden, Arps, Slate, Meagher & Flom LLP
       General Counsel - Corporate Law                       919 Third Avenue
               Citigroup Inc.                            New York, New York 10022
            153 East 53rd Street
          New York, New York 10043
</TABLE>
 
                              TO THE UNDERWRITERS
                            AS TO UNITED STATES LAW
 
                              Dewey Ballantine LLP
                          1301 Avenue of the Americas
                            New York, New York 10019
 
                           AUDITORS TO CITIGROUP INC.
 
                                    KPMG LLP
                                345 Park Avenue
                            New York, New York 10154


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