An end-on superoxido complex with the formula {[Co-III(OH2) (trpy)] [Co-III(OO center dot) (trpy) (mu-bpp)}(4+) (3(4+)) (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 O-17 and O-18 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)(mu-bpp) (mu-OO)}(3+) (1(3+)). DFT calculations agree with and complement the experimental data, offering a complete description of the transition states and intermediates involved in the catalytic cycle.