Tolerance to small amounts of CO is important for proton exchange membrane fuel cells (PEMFCs) operating on hydrogen obtained by reforming carbon-based fuels. Platinum-based catalysts used today suffer high anode polarization losses, reducing performance and fuel efficiency. Here, findings in the search for CO-tolerant catalysts are reported. Novel platinum-based catalysts were discovered that show low polarization for oxidation of hydrogen containing 104 ppm CO, and little adsorbed CO was observed in CO stripping studies. Platinum in these catalysts is believed to have sulfur in its structure obtained from sodium thiosulfate used in the synthesis of the Pt catalysts. The activity of the catalyst varies with the amount of free platinum surface available, with one catalyst showing better activity than the Pt-Ru catalyst. Sulfur present in the catalysts resulted in redox activity at lower potentials than that shown by adsorbed sulfur on Pt and was stable during cyclic potential sweeps. (C) 2004 The Electrochemical Society.