The effect of CO gas and anode-metal loading on H-2 oxidation in a proton exchange membrane fuel cell (PEMFC) are investigated by ac impedance spectroscopy. To investigate these effects, the voltage loss is measured, and the impedance of the half-cell (cathode side: H-2; anode side: simulated gas) and full-cell (cathode side: O-2; anode side: simulated gas) are determined by ac impedance spectroscopy. The CO gas has a great effect on the charge-transfer reaction (high-frequency arc) and hydrogen dissociative chemisorption (medium-frequency are), but little effect on the low-frequency arc. The polarization resistances for charge-transfer and hydrogen dissociative chemisorption in a fuel cell with low metal loading are larger than those with high metal loading, and increase greatly with increasing CO concentration. Although the cathode impedance is the 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.