Photocatalytic decomposition of water has been studied by using TiO2 (P25) and B/TiO2 photocatalysts. Yield of the photocatalytic decomposition of water by TiO2 (P25) depended on direction of light irradiation. The photocatalytic activity was about 10(3) times as high when the reaction cell was irradiated from the top as from the bottom. Modification of TiO2 with boron oxides was found to be very effective for photocatalytic decomposition of water. TiO2 modified with boron oxides exhibited high reactivity for the decomposition of water indifferently to the direction of light irradiation. (C) 2000 Elsevier Science B.V. All rights reserved.