Pt-Cu supported on CeO2 and mechanically mixed with CeO2 were synthesized using an electron beam irradiation method to probe the active metal-oxide interfaces for catalytic CO preferential oxidation. The lack of activity for the mechanical mixture of Pt with CeO2 showed the metal-CeO2 interface is critical for monometallic Pt. The comparable activity for the CeO2-supported Pt-Cu and mechanical mixture of Pt-Cu with CeO2 suggested platinum-copper contact as a new active site for bimetallic Pt-Cu. A non-linear increase of activity along the Cu content in catalyst and the Cu-O bonds detected in XANES spectra in the reaction condition at 100 A degrees C suggested the presence of CuOx on the Pt-Cu alloy surface as strong chemisorption sites for oxygen.