PtAu alloy nanoparticle catalysts for use in direct methanol fuel cells were synthesized by reduction with NaBH4 and freeze-drying and their electrocatalytic activities were examined for reactions of methanol, formaldehyde, and formic acid. The extent of alloy formation and average particle size were characterized by X-ray diffraction and transmission electron microscopy. X-ray photoelectron spectra confirmed that the surface state of Au in the PtAu alloy was exclusively metallic, while the Ru in the alloy has not only metallic characteristics but is also present in oxidized form. Based on electrochemical measurements, the PtAu catalyst showed a more enhanced activity than pure Pt for the oxidation of methanol, having a lower onset potential and a larger current density. The electrocatalytic activity of the PtAu catalyst was also enhanced in the oxidation of formic acid but not formaldehyde. This provides evidence for differences in the catalytic activity of PtAu in the oxidation of low molecular weight organic compounds. The origin of the enhanced catalytic activity of the PtAu catalysts is discussed from the standpoint of a modified methanol oxidation pathway in which formaldehyde, formic acid, and CO are produced as putative intermediates. (c) 2006 The Electrochemical Society.