The mechanism of the interaction of Cu+-a,a'-dipyridyl complex (Cu+Lz) with 0 2 in both neutral and acid media was studied by the stopped-flow method. The dependence of the mechanism on the acidity of the medium was established. In an acid medium H+ participated in a direct 0 2 reduction to HOn by interaction with an oxygen adduct L&u+O2 formed without displacement of ligand molecules. In a neutral medium the reaction rate was limited by inner sphere charge transfer from Cu+ to 0 2 to form an oxygen "charge transfer" complex L2CuOt. The latter interacted either with the second ion Cu+L2 or with the free ligand, or else it dissociated, reversibly or irreversibly, to form a radical anion 0;. The bimolecular rate constants of the oxygen "adduct" and "charge transfer" complex formation appeared to be kbi = (1.0 f 0.1) X lo5 and (1.5 f 0.2) X 104M-'-sec-', respectively. The effective termolecular rate constants of 0 2 reduction to H02 in an acid medium (with contribution from H+) and to 0; in a neutral medium (with contribution from a,a'-dipyridyl) were k,,, = 2.7 X lo8 and 107M-2.sec-1. The rate constants of the elementary steps were estimated. The auto-oxidation mechanism of the aquoion and complexes of Cu+ is discussed in terms of the results obtained.