The CO oxidation activity of Pt deposited on Ta2O5/Ta was studied with various amounts of Au post-deposited on Pt/Ta2O5/Ta. For Pt nanoparticles with a mean size of 2-4 nm, an enhancement in the CO oxidation activity with increasing amount of post-deposited Au was found. The mixed Au-Pt nanoparticles with sizes in the range of 2-4 nm exhibited higher stability than the bare Au nanoparticles with a similar size range. In contrast to the results obtained with the Pt nanoparticles, the catalytic activity of a thicker Pt film gradually decreased with increasing amount of Au deposited. Based on the CO desorption experiments, it is suggested that the surface of the catalytically active Au-Pt bimetallic structures consists of both Au and Pt sites.