Proton exchange membrane fuel cells (PEMFC) show a significant performance drop in CO containing hydrogen as fuel gas in comparison to pure hydrogen. The lower performance is due to CO adsorption at the anode thus poisoning the hydrogen oxidation reaction. Two approaches to improve the cell performance are discussed. First, the use of improved electrocatalysts for the anode, such as PtRu alloys, can significantly enhance the CO tolerance. On the other hand, CO poisoning of the anode could be avoided by the use of non-electrochemical methods. For example, the addition of liquid hydrogen peroxide to the humidification water of the cell leads to the formation of active oxygen by decomposition of H2O2 and the oxidation of CO. In such a way a complete recovery of the CO free cell performance is achieved for H-2/100 ppm CO.