Abstract The performance effect of the hydrogen fuel contaminant carbon monoxide (CO) for dry gas concentrations ranging from 1 to 10ppm in proton exchange membrane fuel cells (PEMFCs) is reported. Performance loss is investigated for different operating conditions including several temperatures, anode relative humidities, and current densities. An analytical method is presented to quantify performance loss due to CO exposure (Δη), the time required to attain steady state of the CO reaction mechanism (ttrans), and the effectiveness of cell performance recovery in neat hydrogen. For select experiments, the magnitude of conversion of CO to carbon dioxide (CO2) at steady state (ξCO) is quantified with gas chromatography. The results show (i) Δη is greatest at high CO concentrations, high current density, and low anode relative humidity, (ii) oxygen crossover from the cathode decreases Δη, and (iii) ξCO is higher at a temperature of 60°C than 80°C. The latter may be attributed to an increased amount of liquid water in the cell increasing the oxygen crossover. The data also shows that recovery in neat hydrogen was typically complete. A summary of all experimental results of Δη is given in Appendix B as fitting equations and parameters to assist in cell performance model development.