The impact of the fuel contaminant CO, which was intentionally injected in to the hydrogen stream at a concentration of 2 ppm, on proton exchange membrane fuel cell (PEMFC) performance distribution was studied using a segmented cell system and spatial electrochemical impedance spectroscopy (EIS). The cell was operated under a galvanostatic control of the overall cell current at 0.8 A cm(-2), and tests were carried out under H-2/O-2, H-2/air, and H-2/H-2 gas configurations. Upon CO injection the voltage decreased by 0.080, 0.300, and 0.320 V for O-2, air, and H-2 cathode gases, respectively. The voltage drop was accompanied by changes in the current density distributions. Inlet segments 1-4 showed a decrease in current due to CO adsorption on Pt, while, downstream segments 7-10 exhibited an increase in current. The performance completely recovered within 1-2 h after CO injection was stopped. The conversion of CO proceeds through a combination of catalytic and electrochemical oxidation reactions; however, the catalytic oxidation of CO is likely the dominant process. It was found that an increased membrane gas permeability can mitigate the impact of CO, mainly due to the catalytic oxidation of adsorbed CO on the Pt anode by the permeated O-2. (C) 2012 Elsevier B.V. All rights reserved.