An apparatus removes CO from a hydrogen-rich gas stream in a hydrogen fuel cell system. CO fouls costly catalytic particles in the membrane electrode assemblies. Both a catalyst adapted to perform a water gas shift reaction, and a carbon dioxide adsorbent are disposed in a rotating pressure swing adsorber housing. The adsorption of carbon dioxide shifts equilibrium toward carbon monoxide consumption. A second adsorbent may be disposed in the housing for adsorbing carbon monoxide at low temperatures, and is adapted to desorb carbon monoxide at high temperatures. The present invention advantageously eliminates a unit operation from a space-constrained fuel cell vehicle by combining the WGS catalyst and a CO.sub.2 adsorbent in a single reactor/housing. The apparatus further eliminates the use of a PROX reactor, by providing an apparatus which incorporates CO.sub.2 adsorption and consequent carbon monoxide consumption in the place of the PROX reactor.