The Co and CO2 poisoning effects on the degradation of cell performance or proton exchange membrane fuel cell (PEMFC) tinder transient stage were investigated. The mechanism of CO poisoning lies in the preferential adsorbing or CO to the platinum surface and the blocking or active sites of hydrogen. These phenomena were described with adsorption, desorption, and electro-oxidation processes of CO and hydrogen in the present work. In addition, it is well known that the reverse water gas shift reaction (RWGS) is the main effect of the CO2 poisoning, through which a large part of the catalytic surface area becomes inactive due to the hydrogen dissociation. The predicted results showed that, by contaminating the fuel with 10 ppm CO at the condition of P-H = 0.8 atm and P-co2 = 0.2 atm, the current density of the PEM fuel cell was lowered 28% with rate constant of RWGS k(rs) from zero to 0.02. With 50 ppm CO, the performance drop was only 18%. For the reformed gas, CO2 poisoning became much more significantly when the CO content in the reactant gas was small. (C) 2008 Elsevier B.V. All rights reserved.