Special catalysts-Pt supported zeolites-for the selective oxidation of carbon monoxide in reformed fuels from methanol or natural gas were proposed. They can be applied for the application to polymer electrolyte fuel membrane cells of which anode Pt catalysts suffer serious poisoning by the presence of trace carbon monoxide. The proposed Pt-supported zeolite catalysts can oxidize carbon monoxide much more selectively in a large excess of hydrogen with the addition of a low concentration of oxygen than a conventional Pt-supported alumina catalyst. The selectivity was affected by supports, in the order A type zeolite>mordenite>X type zeolite>alumina. With decreasing oxygen content, an enhanced selectivity was obtained on Pt-zeolite catalysts (approaching 100%). Pt supported on mordenite showed the highest selectivity, with high conversion from carbon monoxide to carbon dioxide among the catalysts examined. It was demonstrated that the oxygen addition can be minimized almost to the stoichiometric amount required for the complete oxidation of carbon monoxide in a large excess of hydrogen.