The CO2 poisoning effect on anodes for the proton-exchange-membrane fuel cell (PEMFC) was examined by model studies. It was assumed that the reverse water gas shift reaction (RWGS) is the origin of the CO2 poisoning effects. The relation between the anode polarisation losses and the catalytic properties of the catalyst was investigated with a kinetic model and with a fuel cell model including finite utilisation of the fuel. It was found that the main effect of the occurrence of the reverse water gas shift reaction is that a large part of the catalytic surface area becomes inactive for hydrogen dissociation. Desorption of CO formed by reduction of CO2 followed by transport down the anode gas channel and subsequent re-adsorption on the catalyst was shown to play a minor role. In reformate gas, where besides CO2 traces of CO are present, CO2 poisoning will have the largest effect when the CO content is small and at relatively low current density. (C) 2004 Elsevier B.V. All rights reserved.