Trinuclear Ru cluster photocatalysts that contain two Ru(II) photo-sensitizers and a Ru(II) reaction center are prepared, and their activity in the photo-catalytic oxygenation of a sufide and alkenes is investigated. Photoirradiation (visible light) and the use of a Co salt ([Co(NH3)(5)Cl]Cl-2, as an electron acceptor) are found to be essential for these catalytic reactions. The 0 atom of the water solvent (pH 6.8) is transferred to the substrate by a stepwise electron transfer and deprotonation of an aqua ligand at the reaction center. Through these processes, the aqua ligand coordinated at the reaction center is converted to a Ru-V=O species, which is the active intermediate in the sulfide and alkene oxygenation.