Abstract A titanium-boron binary oxide has been prepared by the sol-gel method and used as a photocatalyst for the decomposition of water. The structure of titanium oxide species in the Ti/B binary oxide was amorphous before and crystal after calcination in O2, while the boron oxide species retained its amorphous state. Pt-loaded Ti/B photocatalysts were found to decompose water stoichiometrically in aqueous suspension systems. Their photocatalytic activities decreased markedly with increase in the calcination temperature, indicating that the photocatalytic activity of the Ti/B binary oxide was strongly dependent on the crystal phase of the titanium oxide.