The purpose of this paper was to investigate the effect of gas phase mass transfer and the influence of different reactions on the anode performance and to understand previously made experiments better. This has been done by mathematically modeling how different effects influence the polarization curve of the anode. Some previously obtained experimental data were used as input for the model. In this study, results from using the mechanisms proposed for the hydrogen oxidation by Jewulski and Suski and Ang and Sammels, respectively, show that they are equally likely. Furthermore, the direct electrochemical oxidation of carbon monoxide only slightly influences the anode performance. The concentration gradients in the current collector are larger than inside the electrode for gases not in equilibrium when entering the current collector; this is an effect caused by the shift reaction inside the electrode. However, if the gas compositions correspond to equilibrium at the current collector, the gas composition profiles become almost uniform. The disparities of the partial pressure dependency found in earlier experiments may be explained if the inlet gas composition is assumed to be the one obtained directly after humidification and not in equilibrium, as generally assumed. (c) 2006 The Electrochemical Society.