We studied the influence of CO poisoning of the anode in the polymer electrolyte membrane fuel cell (PEMFC) using electrochemical impedance spectroscopy (EIS). The anode impedance was found by first feeding neat hydrogen gas and next hydrogen with CO into one of the electrodes, keeping neat hydrogen gas on the other electrode as a reference. The electrodes were E-TEK Elat gas-diffusion elec trodes with 0.5 mg Pt/cm(2) and the membrane was Nafiona((R)) 117. The CO concentration was 103 ppm, and the total pressures were 1, 2.5 and 4 bar. Operating temperatures were kept constant, 30.0 degrees C or 50.0 +/- 0.1 degrees C. Bias voltages of 0 and 0.05 V were used. Three steps were revealed in the reaction mechanism, the slow adsorption/diffusion step, the charge transfer step and the proton hydration step, confirming earlier results. Carbon monoxide affects the charge transfer step by blocking active sites and by affecting the surface polarisation. We further conclude that CO adsorbs to the porous carbon matrix, and reduces significantly the rate of surface diffusion of hydrogen to the surface. (c) 2007 Elsevier B.V. All rights reserved.