Catalytic four-electron reduction of O-2 by ferrocene (Fc) and 1,1'-dimethylferrocene (Me(2)Fc) occurs efficiently with a dinuclear copper(II) complex [Cu-2(II)(XYLO)(OH)](2+) (1), where XYLO is a m-xylene-linked bis[(2-(2-pyridyl)ethyl)amine] dinucleating ligand with copper-bridging phenolate moiety], in the presence of perchloric acid (HClO4) in acetone at 298 K. The hydroxide and phenoxo group in [Cu-2(II)(XYLO)(OH)](2+) (1) undergo protonation with HClO4 to produce [Cu-2(II)(XYLOH)](4+) (2) where the two copper centers become independent and the reduction potential shifts from -0.68 V vs SCE in the absence of HClO4 to 0.47 V; this makes possible the use of relatively weak one-electron reductants such as Fc and Me(2)Fc, significantly reducing the effective overpotential in the catalytic O-2-reduction reaction. The mechanism of the reaction has been clarified on the basis of kinetic studies on the overall catalytic reaction as well as each step in the catalytic cycle and also by low-temperature detection of intermediates. The O-2-binding to the fully reduced complex [Cu-2(I)(XYLOH)](2+) (3) results in the reversible formation of the hydroperoxo complex ([Cu-2(II)(XYLO)(OOH)](2+)) (4), followed by proton-coupled electron-transfer (PCET) reduction to complete the overall O-2-to-2H(2)O catalytic conversion.