Anode gas outlet CO concentration is measured by an online infrared gas analyzer for a proton exchange membrane (PEM) fuel cell fed with H-2/100 ppm CO and with Pt or PtRu as anode catalyst. CO concentration increased with anode inlet flow rate at a given current density for Pt catalyst. CO outlet concentration with PtRu catalyst depended upon the current density at a cell temperature of 80degrees C. The CO concentration decreased with the increase of anode inlet flow rate at low current densities, while at higher current densities the outlet CO concentration followed a trend similar to that observed with Pt. The CO electro-oxidation rate on Pt and PtRu is thus calculated using CO material balance on the anode side. Results indicate that the enhanced tolerance of PtRu catalyst is due to the dual mechanisms of reduced CO affinity (ligand effect) and enhanced CO electro-oxidation rates (bifunctional effect), with either mechanism dominating depending on anode overpotential. (C) 2002 The Electrochemical Society.