Catalysts in which Pt and Cu are immobilized on support particles of γ-Fe2O3 were synthesized by the radiolytic process and were evaluated for CO oxidation in a gas flow mixture (1% CO, 0.5% O2, 67.2% H2 and N2 balance) by measuring the CO concentration in the outlet gas. The Pt/Cu atomic ratios of the as-synthesized catalysts were determined to be 100:0, 90:10, 78:22, 50:50, 21:79, and 11:89, and the total metal loadings determined by chemical analyses were 10 wt%. Material characterization was performed using X-ray diffraction, X-ray absorption near edge structure, and transmission electron microscopy, and it was indicated that the composite catalysts consist of Pt-Cu bimetallic grains immobilized on the support at higher Pt-loading, while CuO with poor crystallinity is also observed at lower Pt-loading. The catalytic activity decreased as the Pt-loading was decreased to 50 at%, and also with increasing temperature. However, as the Pt-loading was further decreased, the activity contrariwise increased, and increased with increasing temperature up to 100 °C. The sample containing only 11 at% Pt exhibited the highest activity at 100 °C, which is higher than that of the commercial catalyst measured for comparison, and given at a lower temperature than that for the commercial catalyst. This enhanced activity, despite the low Pt-loading, could be attributed to oxygen supply via CuO from the O2-poor atmosphere to PtCu bimetallic grains trapping CO molecules. This new material is promising for use as a catalyst to purify hydrogen gas fed to a polymer electrolyte fuel cell.