Assessment of CO-tolerance for different anode catalysts (Pt/C, PtRu/C and PtSn/C) at 50 degreesC and PtRu/C at different operating temperatures ill the polymer electrolyte fuel cell (PEFC) was evaluated using ac impedance spectroscopy. Impedance measurements for the oxidation of a H-2 + 2% CO mixture reported that at a critical potential (referred to as the onset potential), 'pseudo-inductive' behavior appears, the value of which is different for different anode catalysts (0.70, 0.45 and 0.30 V for Pt/C, PtRu/C and PtSn/C at 50 degreesC, respectively) and strongly dependent upon working temperatures (0.50, 0.45 and 0.40 V for PtRu/C at 25, 50 and 70 degreesC, respectively). Based on that, we proposed that this onset potential for the 'pseudo-inductive' behavior can be used as a new criterion for the evaluation of CO-tolerance for different anode catalysts at different operating temperatures.