Abstract: We have recently proposed special catalysts, Pt-supported zeolites, for the selective oxidation of carbon monoxide in reformed fuels from methanol or natural gas. They can be applied to polymer electrolyte fuel cells, of which Pt anode catalysts suffer serious poisoning by the trace amount of CO. In order to enhance the performance of the catalysts, the selective CO oxidation was examined over Pt—, Ru—, Pd—, Co—, and PtRu— supported zeolites in a large excess of H2 with the addition of a low concentration of oxygen. It was found that Pt, Ru, and PtRu supported on mordenite can oxidize CO much more selectively than a conventional Pt-supported alumina catalyst. Ru/mordenite showed the highest selectivity of approximately 100% at 200° C. with a certain flow rate of the reactant. Pt-Ru/mordenite exhibited fairly high selectivity of 90% over a wide flow rate condition even at 150° C.