The complexes trans-Ru(porp) (O)2, where porp = the dianion of 5,10,15,20-tetramesitylporphyrin (TMP) or 5,10,15,20-tetra(2,6-dichlorophenyl)porphyrin (OCP), are readily formed in benzene by treatment of the Ru(II) bis(acetonitrile) precursors with O2 or air. Such dioxo species in solution utilize both oxygen atoms for oxygenation of thioethers to sulfoxides, phenol to hydroquinone, and (as noted by other groups) olefins to epoxides; 2-propanol is also dehydrogenated to give acetone. Catalytic O2-oxygenation has been demonstrated for the thioether and olefinic substrates. In summary, dioxo(porphyrinato)ruthenium(VI) species are capable of transferring O-atoms to, or abstracting H-atoms from, several diverse types of substrates; as O2 is the O-atom source, the systems represent a major advance in O2-oxidation chemistry and offer an excellent opportunity for detailed mechanistic insight into oxidations of biological and industrial importance. The scene is comparable to that of catalytic hydrogenation in the early 1960's, when many transition metal hydrides were being synthesized using H2, and their catalytic properties were being discovered (ref. 43).