The kinetics of 1 : 1 electron-transfer reactions between the µ-amido-µ-peroxo-complex, (en)₂Co·µ(NH₂,O₂)·Co(en)₂³⁺, and four µ-superoxo-complexes have been studied. All four reactions proceed to >90% completion under the conditions employed, and the unprotonated B-form of the µ-amido-µ-peroxo-complex with bonding Co–O–O–Co is at least predominantly the involved reactant. At 25°, µ= 0·245M, rate constants (l mol^–1 s^–1) are respectively for the reaction of B with (NH₃)₅Co·O₂·Co(NH₃)₅⁵⁺, 0·170 ± 0·013; (NH₃)₄Co·µ(NH₂,O₂)·Co(NH₃)₄⁴⁺, 12·0 ± 0·85; (bipy)₂Co·µ(NH₂,O₂)·Co(bipy)₂⁴⁺, 2·35 (±0·19)× 10³; and (phen)₂ Co·µ(NH₂,O₂)·Co(phen)₂⁴⁺, 6·17 (±1·44)× 10³. The first two reactions were studied in perchlorate media and the second two in nitrate media. From data at tow temperatures activation parameters for the reaction of (NH₃)₅Co·O₂·Co(NH₃)₅⁵⁺ are ΔH^‡∼ 13·2 kcal mol^–1 and ΔS^‡∼–18 e.u. Anions Cl^–, SO₄^2–, and NO₃^– have only a mild catalytic effect on this same reaction. Comparisons with data for the one-equivalent reductions of the µ-superoxo-complexes with Fe²⁺ are made.