The influence of the direction of ultraviolet irradiation upon the yields of the photocatalytic decomposition of pure water has been investigated in Pt/TiO2 aqueous suspension system. The yields of the photocatalytic decomposition irradiated from the top of the reaction cell are about 10(3) times higher than those irradiated from the bottom. The difference seems due to the reverse reaction of the formed H-2 and O-2 in the suspension.