Visible light irradiation of a reaction mixture of carbonyl-coordinated tetra(2,4,6-trimethyl)phenylporphyrinatoruthenium(II) ((RuTMP)-T-II(CO)) as a photosensitizer, hexachloroplatinate(IV) as an electron acceptor, and an alkene in alkaline aqueous acetonitrile induces selective epoxidation of the alkene with high quantum yield (Phi = 0.6, selectivity = 94.4% for cyclohexene and Phi = 0.4, selectivity = 99.7% for norbornene) under degassed conditions. The oxygen atom of the epoxide was confirmed to come from a water molecule by an experiment with (H2O)-O-18. cis-Stilbene was converted into its epoxide, cis-stilbeneoxide, without forming trans-stilbeneoxide. trans-Stilbene, however, did not exhibit any reactivity. Under neutral conditions, an efficient buildup of the cation radical of (RuTMP)-T-II(CO) was observed at the early stage of the photoreaction, while an addition of hydroxide ion caused a rapid reaction with the cation radical to promote the reaction with reversion to the starting (RuTMP)-T-II(CO). A possible involvement of a higher oxidized state of Ru such as Ru-IV, Ru-V, Ru-VI through a dismutation of the Ru-III species was excluded by an experiment with (RuTMP)-T-VI(O)(2). Decarbonylation of the Ru complex was also proven to be invalid. A reaction mechanism involving an electron transfer from the excited triplet state of (RuTMP)-T-II(CO) to hexachloroplatinate(IV) and subsequent formation of OH--coordinated Ru-III species, leading to an oxo-ruthenium complex as the key intermediate of the photochemical epoxidation, was postulated.