Catalytic water oxidation to generate oxygen was achieved using all-inorganic mononuclear ruthenium complexes bearing Keggin-type lacunary heteropolytungstate, [Ru III(H 2O)SiW 11O 39] 5- (1) and [Ru III(H 2O)GeW 11O 39] 5- (2), as catalysts with (NH 4) 2[Ce IV(NO 3) 6] (CAN) as a one-electron oxidant in water. The oxygen atoms of evolved oxygen come from water as confirmed by isotope-labeled experiments. Cyclic voltammetric measurements of 1 and 2 at various pH's indicate that both complexes 1 and 2 exhibit three one-electron redox couples based on ruthenium center. The Pourbaix diagrams (plots of E 1/2 vs pH) support that the Ru(III) complexes are oxidized to the Ru(V)-oxo complexes with CAN. The Ru(V)-oxo complex derived from 1 was detected by UV-visible absorption, EPR, and resonance Raman measurements in situ as an active species during the water oxidation reaction. This indicates that the Ru(V)-oxo complex is involved in the rate-determining step of the catalytic cycle of water oxidation. The overall catalytic mechanism of water oxidation was revealed on the basis of the kinetic analysis and detection of the catalytic intermediates. Complex 2 exhibited a higher catalytic reactivity for the water oxidation with CAN than did complex 1.