The effects of coexistent compounds, such as H2O and CO2, contained in hydrocarbon reformed gas on CO preferential oxidation over a Ru catalyst were investigated for residential polymer electrolyte fuel cell (PEFC) systems using the natural gas steam reforming process. The presence of either or both H2O and CO2 decreased the CO preferential oxidation removal performance of the Ru catalyst over all temperature ranges. Consequently, the temperature window of the catalyst for reducing CO to less than 10 ppm was narrowed by those compounds.