NOVELTY - In an isothermal reactor, more of the CO is reacted with O2, and less CO is formed in the reverse reaction than in a similarly-sized heat exchanger operating at the same temperature, but without the mixing chamber.    USE - Used for preferential oxidation of carbon monoxide in a hydrogen-rich fuel stream for a fuel cell.    ADVANTAGE - Provides efficient, multi-stage reactor that periodically homogenizes the gas being treated en route through the reactor to uniformly distributes the CO and O2 in the gas between different reaction stages of the reactor.    DETAILED DESCRIPTION - In an isothermal reactor for the selective reaction of O2 with CO in a H2-rich gas to form CO2 in the gas, the reactor comprises a catalyzed heat exchanger having thermally conductive barriers separating the heat exchanger into first channels through which separate streams of the gas flow and second channels through which a coolant flows. The first channels have inlet ends for admitting the gas into the first channels and outlet ends for exhausting the gas from the first channels. The barriers each have (a) catalyzed first surface confronting the first channel for promoting the reaction and (b) a second surface confronting the second channel for contacting the coolant in the second channel to extract heat from the catalyzed first surface and maintain the heat exchanger at a constant temperature suitable to promoting the reaction and discouraging the formation of CO from the reverse reaction of CO2 with H2. The improvement comprises at least one mixing chamber communicating with the first channels intermediate the inlet and outlet ends and dividing the heat exchanger into at least one heat exchanger segment upstream of the chamber and at least one heat exchanger segment down stream of the chamber in the direction of the streams flow through the first channels. The chamber is adapted to receive and homogenize the streams exiting the upstream segment so as to distribute any unreacted O2 and CO in the streams uniformly throughout the has before the gas enters the downstream segment of the heat exchanger. An INDEPENDENT CLAIM is included for a multistage isothermal reactor.