The once reported Ag-modified CaTiO3 photocatalyst was reexamined by optimizing the Ag loading amount and using a conventional photochemical reactor. This revealed that the Ag-modified CaTiO3 photocatalyst actually showed both high production rate of CO (54 mu mol h(-1)) and excellent selectivity towards CO formation (94%) by suppressing the H-2 production via water splitting. It is suggested that the high photocatalytic performance originates from not only the optimized amount of cocatalyst and the high irradiation light intensity but also the high concentration of dissolved CO2 that was achieved by a bubbling flow of CO2 at the lower reaction temperature. These reaction conditions provided ca. 40 times higher CO formation rate. It was proposed that the deposited small Ag nanoparticles are the selective active sites for CO formation and the CaTiO3 crystal surface produces H-2 preferably.