The performance of a proton exchange membrane fuel cell (PEMFC) was evaluated at 80 degrees C in H-2 with defined amounts of CO (25 to 250 ppm) and pure oxygen. Membrane electrode assemblies (MEAs) were made using Nafion(TM) 117 with carbon-supported dispersed Pt, Pt0.7Ru0.3, and Pt0.5Ru0.5 as anode catalysts, and Pt as a cathode catalyst. For comparison the MEAs were first characterized in H-2//O-2. In H-2/CO//O-2 steady-state current-voltage curves were obtained after a poisoning period. It was found that the performance of the cell depends strongly on the do concentration and the anode catalyst used. For Pt0.5Ru0.5 as anode catalyst, the maximum power density in H-2/CO//O-2 was enhanced by a factor of four compared with a Pt anode catalyst. Using Pt0.5Ru0.5 no difference in power density was found between pure H-2 and H-2/100 ppm CO up to current densities of about 0.4 A cm(-2).