High-valent oxo compounds of transition metals are often implicated as active species in oxygenation of hydrocarbons through carbonhydrogen bond activation or oxygen transfer and also in water oxidation. Recently, several examples of cobalt-catalyzed water oxidation have been reported, and cobalt(IV) species have been suggested as active intermediates. A reactive species, formally a dicobalt(IV)-mu-oxo polyoxometalate compound [(alpha(2)-P2W17O61Co)(2)O](14-), [(POMCo)(2)O], has now been isolated and characterized by the oxidation of a monomeric [alpha(2)-P2W17O61CoII(H2O)](8), [(POMCoH2O)-H-II], with ozone in water. The crystal structure shows a nearly linear Co-O-Co moiety with a CoO bond length of similar to 1.77 A. In aqueous solution [(POMCo)(2)O] was identified by P-31 NMR, Raman, and UV-vis spectroscopy. Reactivity studies showed that [(POMCo)(2)O](2)O] is an active compound for the oxidation of H2O to O-2, direct oxygen transfer to water-soluble sulfoxides and phosphines, indirect epoxidation of alkenes via a Mn porphyrin, and the selective oxidation of alcohols by carbon-hydrogen bond activation. The latter appears to occur via a hydrogen atom transfer mechanism. Density functional and CASSCF calculations strongly indicate that the electronic structure of [(POMCo)(2)O](2)O] is best defined as a compound having two cobalt(III) atoms with two oxidized oxygen atoms.