NOVELTY - A carbon monoxide-selective catalyst comprises a catalytic material consisting of platinum, palladium, rhodium, iridium, osmium, ruthenium, tantalum, zirconium, yttrium, cerium, nickel, copper, their oxides, alloys, and/or compounds; and a modifying agent (2-25 atom%) consisting of lead, bismuth, germanium, silicon, antimony, arsenic, phosphorus, their, alloys, nitrates, sulfides, and/or chlorides.    USE - Useful for a fuel cell system (claimed).    ADVANTAGE - The invention is preferentially selective to carbon monoxide over hydrogen, and can be used up to 600, preferably up to 400degreesC, thus eliminating the need for a sophisticated process control system, and thus resulting in a simpler, more efficient, and less expensive fuel processing system. It reduces carbon monoxide content to e.g. concentration of less than 500, preferably less than 10 ppm, without creating polluting by-products such as methane.    DETAILED DESCRIPTION - A carbon monoxide (CO)-selective catalyst comprises a catalytic material consisting of platinum, palladium, rhodium, iridium, osmium, ruthenium, tantalum, zirconium, yttrium, cerium, nickel, copper, their oxides, alloys, and/or compounds; a modifying agent (2-25 atom%) consisting of lead, bismuth, germanium, silicon, antimony, arsenic, phosphorus, their, alloys, nitrates, sulfides, and/or chlorides, where the atom% is the number of atoms of the modifying agent present in 100 surface atoms of the catalytic material; and a support.    INDEPENDENT CLAIMS are also included for:    (a) a method of forming a CO-selective catalyst comprising combining a catalytic material, and a support with 2-25 atom% of a modifying agent, based on the total surface atoms of the catalytic material, to form a modified catalyst-containing support; and disposing the modified catalyst-containing support on or into a substrate; and    (b) a fuel cell system comprising a fuel reformer; a CO-selective reformer in fluid communication with the fuel reformer, and comprising a housing disposed around a substrate comprising a CO-selective catalyst; and a fuel cell in fluid communication with the CO-selective reformer.    DESCRIPTION OF DRAWING(S) - The figure is a graph illustrating the CO oxidation rates over the lead-containing platinum catalyst, and the platinum-only catalyst supported on zirconia in a CO and O2 mixture.