The influence of CO2, CO and air bleed on current distribution was studied during transient operation, and the dynamic response of the fuel cell was evaluated. CO causes significant changes in the current distribution in a polymer electrolyte fuel cell. The current distribution reaches steady state after approximately 60 min following addition of 10 ppm CO to the anode fuel stream. Air bleed may recover the uneven current distribution caused by CO and also the drop in cell voltage due to CO and CO2 poisoning. The recovery of cell performance during air bleed occurs evenly over the electrode surface even when the O-2 partial pressure is far too low to fully recover the CO poisoning. The O-2 supplied to the anode reacts on the anode catalyst and no O-2 was measured at the cell outlet for air bleed levels up to 2.5%. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.