For the decomposition of pure water to hydrogen and oxygen, MIn2O4 (M = Ca, Sr), Sr2SnO4, and NaSbO3 were found to make stable photocatalysts when combined with RuO2. The metal oxides are composed of the d(10) p-block metal ions (In3+, Sn4+, and Sb5+), and it is demonstrated that the p-block metal oxides with d(10) configuration form a new photocatalyst group different from the conventional metal oxide photocatalyst group consisting of the octahedrally coordinated d(0) transition-metal ions such as Ti4+, Zr4+, Nb5+, and Ta5+.