Carbon monoxide is a conventional contaminant in the fuel obtained from reforming processes with an important influence on the performance of a proton exchange membrane fuel cell (PEMFC). The studies of transient and continuous injection of CO presented here give information about poisoning and recovery processes, and recommend strategies for fuel cell operation. Pulsing study shows that up to 100ppm CO, has no significant effect on the performance. Constant current demand experiments show an oscillatory effect due to CO electro-oxidation at high over-potentials. In continuous poisoning process, kinetic and mass transfer affect the rate of CO removal. To recover the performance for continuous fuel cell operation, we propose cyclic feeding of hydrogen containing traces of CO (i.e. supplied by a reforming process) and pure hydrogen streams. (c) 2005 Elsevier B.V. All rights reserved.