The photocatalytic preferential oxidation of CO with O-2 in the presence of H-2 (photo-PROX) was found to proceed efficiently on the NiO-loaded TiO2 (NiO/TiO2) catalyst under UV light irradiation at 293 K. NiO/TiO2 exhibited higher CO conversion as well as CO2 selectivity for a photo-PROX reaction than the original unloaded TiO2 (P-25). Various spectroscopic investigations have revealed that the small NiO clusters formed on TiO2 play an important role in the enhancement of CO oxidation activity in this reaction.