Using the high-pressure pulse radiolysis technique, we have shown that the oxidation of (phen)2CuI (phen = 1,10-phenanthroline) by molecular oxygen proceeds via a transient in which a copper-oxygen bond is formed, i.e., via CuI-O2 as an intermediate. The volume of activation measured for this process is -22 ± 2 cm3 mol-1. When high concentrations of (phen)2CuII are present in the solution, this transient reacts with another (phen)2CuI yielding (phen)2CuII and H2O2. If the latter reaction involves the formation of CuIO2CuI as an intermediate, it is produced with a rate constant close to the diffusion-controlled limit. In the presence of low concentrations of (phen)2CuII, CuI-O2 decomposes to (phen)2CuII and O2-, where subsequently O2- reacts with another (phen)2CuI to produce (phen)2CuII and H2O2.