Bimetallic composite nanoparticles of PtM (M = Au, Cu or Ni) supported on gamma-Fe2O3 were synthesized by a radiolytic method employing a 4.8-MeV electron beam to reduce aqueous ions. These composite particles were obtained at room temperature just by irradiating glass vials containing source materials in an aqueous system without using any surfactant, organic solvent and stabilizer. The irradiation, taking several seconds, formed metallic nanograins of 3nm in diameter stabilized on the support of gamma-Fe2O3 particle of 13 nm in diameter. XRD patterns of PtAu and PtCu samples showed clear peak shifts indicating occurrence of random alloy phase. X-ray absorption fine structure analysis for the PtCu and PtNi samples indicated significant Pt-M coordination. Catalytic activity of CO oxidation of the samples was evaluated by measuring residual CO contents in air in contact with the sample material with a gas-chromatogra ph. All the PtM systems exhibited activities higher than those of monolithic Pt on gamma-Fe2O3. The correlation between the atomic structures and the catalytic activities indicated that the random alloy structure enhances the activity. (C) 2010 Elsevier B.V. All rights reserved.