The structure of bimetallic nanoparticles has a great impact on catalytic performance. By radiation-induced reduction of metal ions in the aqueous phase, supported bimetallic Pt–Cu catalysts were synthesized with various Cu loadings, support materials, and copper sources. These parameters had a great impact on the structural properties of Pt–Cu nanoparticles. On carbon black, Pt–Cu alloy nanoparticles were readily formed with little oxide phase. On CeO2 support, Pt–Cu was highly oxidized for the low-Cu region (Cu/Pt 1). The structure of CeO2-supported catalysts obtained from two different copper sources (CuSO4 and CuCl2) suggested that such a drastic change in oxidation state is the result of two competing effects, CeO2 to oxygenate metals and the sulfate ion to stabilize them in the metallic state. The reaction characteristics in preferential CO oxidation reflected the Pt–Cu structure (oxidation state) dete...