The rates of establishment of coordination equilibria in solutions containing 2,9-dimethyl-1,10-phenanthroline (DMP) and Cu(II) when the latter is reduced to Cu(I) at electrodes were measured by means of cyclic voltammetry. Formal potentials for the Cu(DMP)(2+/+) and CU(DMP)(2)(2+/+) couples were evaluated (0.08 and 0.35 V vs SCE, respectively) from which the equilibrium constants for the formation of the Cu(DMP)(+) and Cu(DMP)(2)(+) complexes were calculated (6.7 x 10(8) M(-1) and 1.5 x 10(10) M(-2), respectively). Cu(DMP)(2)(+) is not oxidized by O-2, but Cu(DMP)(+), generated by addition of Cu2+ to solutions of Cu(DMP)(2)(+), is oxidized by O-2. A rate law is presented that accounts for the observed second-order dependences of the reaction rate on [Cu2+], [Cu(DMP)(2+)], and [Cu(DMP)(2)(+)](-1). The stoichiometric reduction of O-2 by Cu(DMP)(+) in solution is contrasted with the catalytic reduction of O-2 by the same complex adsorbed on graphite electrodes.