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The effective utilization of solar energy requires photocatalytic reactions with high quantum efficiency. Water is the most abundant reactant that can be used as an oxygen source in efficient photocatalytic reactions, just as nature uses water in an oxygenic photosynthesis. We report that photocatalytic oxygenation of organic substrates such as sodium p-styrene sulfonate occurs with nearly 100% quantum efficiency using manganese(III) porphyrins as an oxygenation catalyst, [Ru(II)(bpy)(3)](2+) (bpy = 2,2'-bipyridine) as a photosensitized electron-transfer catalyst, [Co(III)(NH(3))(5)Cl](2+) as a low-cost and weak one-electron oxidant, and water as an oxygen source in a phosphate buffer solution (pH 7.4). A high-valent manganese-oxo porphyrin is proposed as an active oxidant that effects the oxygenation reactions.