An end-on superoxido complex with formula {[CoIII(OH2)(trpy)][CoIII(OO·)(trpy)](m-bpp)}4+ , 34+, (bpp- is bis-2-pyridyl-3,5-pyrazolate; trpy is 2,2’;6’:2”-terpyridine) has been characterized by resonance Raman, electron paramagnetic resonance and x-ray absorption spectroscopies. These results together with on-line mass spectrommetry experiments using 17O and 18O isotopically labeled compounds prove that this compound is a key intermediate of the water oxidation reaction catalyzed by the peroxido bridging complex {[CoIII(trpy)]2(m-bpp)(m-OO)}3+, 13+. Density Functional Theory calculations agree and complement the experimental data, and offer a complete description of the transition states and intermediates involved in the catalytic cycle