Hydrogen fuel produced by reforming contains, in addition to hydrogen, CO2, CO, sulphur compounds and small hydrocarbons in varying amount. The effect of CO2 and a few example hydrocarbons has been studied on traditional PEM fuel cell anodes by cyclic voltammetry and constant load experiments. In addition, Electrochemical Impedance Spectroscopy and Mass Spectrometry have been employed to aid in the analysis. The effect of adsorption potential, cell temperature, humidity of the feed gases and concentrations of selected impurities has been studied.