The effect of selected impurities that can be brought into the system from the synthesis of the catalyst or from the Nafion membrane on the CO electrooxidation is investigated. In the presence of halide ions, the CO oxidation peak is shifted to more positive values. The extent of displacement is the largest in the case of iodine, followed by bromide, chloride and fluoride. The same order is known for adsorption affinity of halide ions on platinum. Higher halide ion concentrations also result in larger CO peak displacement. All findings are consistent with the assumption that halide ions are adsorbed on sites that are usually occupied by water-the crucial species needed for CO oxidation. Nafion itself seems to have no effect on CO oxidation. The usually observed shifts of CO peak in Nafion-treated surfaces are ascribed to the presence of organic impurities in Nafion. (c) 2005 Elsevier B.V. All rights reserved.