NOVELTY - Preferential oxidation reactor for reducing carbon monoxide (CO) content of gas stream containing hydrogen comprises at least two reactor sections, each having gas passage in flow communication with each other and with a surface supporting a catalyst to promote oxidation of CO in the gas stream, and control mechanism for maintaining temperatures within the gas passages.    USE - The preferential oxidation reactor is used for reducing the carbon monoxide content of a gas stream which contains hydrogen. It is used in a fuel cell system (claimed).    ADVANTAGE - The assembly enables quick light-off of the reactor, while limiting a reverse water-gas shift reaction.    DETAILED DESCRIPTION - Preferential oxidation reactor for reducing carbon monoxide (CO) content of a gas stream which contains hydrogen comprises:    (i) at least two reactor sections, the first of which has a first gas passage through which the gas stream flows and a surface (50) supporting a first catalyst for promoting oxidation of CO in the gas stream, and the second reactor section has a second gas passage in flow communication with the first gas passage and has a surface (52) supporting a second catalyst; and    (ii) control mechanism for maintaining a first temperature within the first gas passage (46) at a value different from the second temperature within the second gas passage (48).    The first catalyst promotes oxidation of the CO at a different rate at the first temperature as compared to the rate at which the second catalyst is operable at the first temperature to oxidize the CO.    INDEPENDENT CLAIMS are also included for:    (1) a fuel cell system comprising a preferential oxidation reactor; and    (2) a method for treating a hydrogen-containing reformate stream within a preferential oxidation reactor to reduce the CO content of the reformate stream, which comprises:    (a) reacting a first portion of CO in the reformate stream at a first temperature in the presence of a first catalyst within a first section of the reactor to oxidize the first portion of CO;    (b) directing the reformate stream from the first reactor section to a second reactor section;    (c) reacting a second portion of CO in the reformate stream in the second reactor section in the presence of a second catalyst at a second temperature to oxidize a second portion of the CO; and    (d) maintaining the second temperature lower than the first temperature.    DESCRIPTION OF DRAWING(S) - The figure is a schematic view of stacked substrate members of the preferential oxidation reactor.    First gas passage (46)    Convoluted element (47)    Second gas passage (48)    Support surfaces (50, 52)