Selective two-electron plus two-proton (2e(-)/2H(+)) reduction of O-2 to hydrogen peroxide by ferrocene (Fc) or 1,1'-dimethylferrocene (Me(2)Fc) in the presence of perchloric acid is catalyzed efficiently by a mononuclear copper(II) complex, [Cu-11(tepa)](2+) (1; tepa = tris[2-(2-pyridyl)ethyl]amine) in acetone. The E-1/2 value for [Cu-11(tepa)](2+) as measured by cyclic voltammetry is 0.07 V vs Fc/Fc(+) in acetone, being significantly positive, which makes it possible to use relatively weak one-electron reductants such as Fc and Me(2)Fc for the overall two-electron reduction of O-2. Fast electron transfer from Fc or Me(2)Fc to 1 affords the corresponding Cu-1 complex [Cu-1(tepa)](+) (2), which reacts at low temperature (193 K) with O-2, however only in the presence of HClO4, to afford the hydroperoxo complex [Cu-11(tepa)(OOH)](+) (3). A detailed kinetic study on the homogeneous catalytic system reveals the rate-determining step to be the O-2-binding process in the presence of HClO4 at lower temperature as well as at room temperature. The O-2-binding kinetics in the presence of HClO4 were studied, demonstrating that the rate of formation of the hydroperoxo complex 3 as well as the overall catalytic reaction remained virtually the same with changing temperature. The apparent lack of activation energy for the catalytic two-electron reduction of O-2 is shown to result from the existence of a pre-equilibrium between 2 and O-2 prior to the formation of the hydroperoxo complex 3. No further reduction of [Cu-11(tepa)(OOH)](+) (3) by Fc or Me(2)Fc occurred, and instead 3 is protonated by HClO4 to yield H2O2 accompanied by regeneration of 1, thus completing the catalytic cycle for the two-electron reduction of O-2 by Fc or Me(2)Fc.