The CO poisoning effect on carbon-supported platinum catalysts (at a loading of 0.5 mg Pt/cm(2) per electrode! in polymer electrolyte membrane fuel cells (PEMFCs) has been investigated in a temperature range from 125 to 200 degreesC with the phosphoric acid-doped polybenzimidazole membranes as electrolyte. The effect is very temperature-dependent and can be sufficiently suppressed at elevated temperature. By defining the CO tolerance as a voltage loss less than 10 mV, it is evaluated that 3% CO in hydrogen can be tolerated at current densities up to 0.8 A/cm(2) at 200 degreesC, while at 125 degreesC 0.1% CO in hydrogen can be tolerated at current densities lower than 0.3 A/cm(2). For comparison, the tolerance is only 0.0025% CO (25 ppm) at 80 degreesC at current densities up to 0.2 A/cm(2). The relative anode activity for hydrogen oxidation was calculated as a function of the CO concentration and temperature. The effect of CO2 in hydrogen was also studied. At 175 degreesC, 25% CO2 in the fuel stream showed only the dilution effect. (C) 2003 The Electrochemical Society.