Stoichiometric electron-transfer (ET) oxidation of two diastereomeric mu-peroxo-mu-hydroxo dinuclear Co(III) complexes with tris(2-pyridylmethyl)amine (TPA) was examined to scrutinize the reaction mechanism of O-2 evolution from the peroxo complexes, as seen in the final step in water oxidation by a Co(III)-TPA complex. The two isomeric Co(III)-peroxo complexes were synthesized and selectively isolated by recrystallization under different conditions. Although cyclic voltammograms of the two isomers in aqueous solutions showed one reversible wave at 1.1 V vs NHE at pH 2.0, two oxidation waves were observed at 1.0 and 1.4 V at pH 7.0 in the aqueous solutions, the latter of which is responsible for the O-2-releasing process. At pH 7, one diastereomer showed higher reactivity than the other in O-2 evolution, indicating the importance of structures of the p-peroxo complexes in the reaction. In order to clarify the O-2-evolving mechanism, we performed electron paramagnetic resonance (EPR) and resonance Raman (RR) measurements for characterizing one-electron oxidized species: The observed EPR and RR signals supported the formation of p-superoxo-p-hydroxo dinuclear Co(III) complexes; however, no characteristic difference was observed between two isomers in the EPR parameters including g values and superhyperfine coupling constants. ET-oxidation rate constants of the isomers were determined to be much faster than the O-2-evolving rate constants, indicating that the O-2-releasing step is the rate-determining step in the O-2 evolution through the stoichiometric ET oxidation of the dinuclear Co(III)-p-peroxo complexes. Therefore, the difference of reactivity in the O-2 evolution for the two isomers should be derived from the thermodynamic stability of two electron oxidized species of the dinuclear Co(III)-it-peroxo complexes, p-dioxygen-y-hydroxo dinuclear Co(III) intermediates.