In order to acquire information regarding the poisoning of Pt catalysts by CO at the anode in polymer electrolyte fuel cells, CO adsorption on Pt electrodes at CO contents of 10-100 ppm in H-2 gas and the electrocatalytic oxidation of H-2 at Pt electrodes covered with CO were investigated with a rotating disk electrode. The equilibrium coverage of Pt with CO reached 1.0 at only 10 ppm CO content. The type of CO bonding on Pt sites was dependent on the CO coverage. The current density for H-2 oxidation at a Pt electrode covered with CO decreased linearly with increasing CO coverage. However, neither the partial pressure of CO in H-2 nor the adsorption type of CO was a direct factor controlling the activity for H-2 oxidation; the CO coverage on the Pt sites was the only crucial factor for the H-2 oxidation in the presence of trace amounts of CO.