The photocatalytic activities of R3MO7 and R2Ti2O7 (R = Y, Gd, La; M = Nb, Ta) strongly depended on the crystal structure. Overall, photocatalytic water splitting into H-2 and O-2 proceeded over La3TaO7 and La3NbO7, which have an orthorhombic weberite structure, Y2Ti2O7 and Gd2Ti2O7, which have a cubic pyrochlore structure, and La2Ti2O7, which has a monoclinic perovskite structure. All of these materials are composed of a network of corner-shared octahedral units of metal cations (TaO6, NbO6, or TiO6); materials without such a network were inactive. The octahedral network certainly increased the mobility of electrons and holes, thereby enhancing photocatalytic activity.