The CO tolerance of a proton exchange membrane fuel cell (PEMFC) was investigated by ac impedance spectroscopy. The impedance of the fuel cell could be obtained by feeding oxygen into the cathode side and simulated gas into anode side. Furthermore, the anode impedance could be obtained by feeding hydrogen into the cathode side and simulated gas into anode side. The CO gas had a greater effect on the charge transfer reaction thigh frequency are) and hydrogen dissociative chemisorption (medium frequency are) but little effect on the low frequency are. Although the cathode impedance is a main part at high temperature, irrespective of CO concentration (less than or equal to 100 ppm), the impedance of the full cell depends on anode impedance at low temperature and high CO concentration. It was found that CO gas has little effect on cathode impedance.