A totally homogeneous, molecular, visible-light-driven water oxidation system is reported. The three system components are (i) a water oxidation catalyst, 1 (a (Ru4O4)-O-IV cluster stabilized by oxidatively resistant [SiW10O32](8-) ligands); (ii) a photosensitizer, [Ru(bpy)(3)](2+); and (iii) a sacrificial electron acceptor, S2O82-. Dioxygen is formed rapidly with an initial turnover frequency of similar to 8 x 10(-2) s(-1) and an estimated quantum yield (defined as the number of O-2 molecules formed per two photons absorbed) of similar to 9%.