An end-on superoxido complex with the formula {[Co^III(OH₂)(trpy)][Co^III(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 {[Co^III(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.