An end-on superoxido complex with the formula {[CoIII(OH2)(trpy)][CoIII(OO•)(trpy)](μ-bpp)}4+ (34+) (bpp- = bis(2-pyridyl)-3,5-pyrazolate; trpy = 2,2';6':2″-terpyridine) has been characterized by resonance Raman, electron paramagnetic resonance, and X-ray absorption spectroscopies. These results together with online mass spectrometry 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-bridged complex {[CoIII(trpy)]2(μ-bpp)(μ-OO)}3+ (13+). DFT calculations agree with and complement the experimental data, offering a complete description of the transition states and intermediates involved in the catalytic cycle.