The selective two-electron reduction of O-2 by one-electron reductants such as decamethylferrocene (Fc*) and octamethylferrocene (Me(8)Fc) is efficiently catalyzed by a binuclear Cu(II) complex [Cu-2(II)(LO)(OH)](2+) (D1) {LO is a binucleating ligand with copper-bridging phenolate moiety} in the presence of trifluoroacetic acid (HOTF) in acetone. The protonation of the hydroxide group of [Cu-2(II)(LO)(OH)](2+) with HOTF to produce [Cu-2(II)(LO)(OTF)](2+) (D1-OTF) makes it possible for this to be reduced by 2 equiv of Fc* via a two-step electron-transfer sequence. Reactions of the fully reduced complex [Cu-2(I)(LO)](+) (D3) with O-2 in the presence of HOTF led to the low-temperature detection of the absorption spectra due to the peroxo complex [Cu-2(II)(LO)(OO)] (D) and the protonated hydroperoxo complex [Cu-2(II)-(LO)(OOH)](2+) (D4). No further Fc* reduction of D4 occurs, and it is instead further protonated by HOTF to yield H2O2 accompanied by regeneration of [Cu-2(II)(LO)(OTF)](2+) (D1-OTF), thus completing the catalytic cycle for the two-electron reduction of O-2 by Fc*. Kinetic studies on the formation of Fc(*+) under catalytic conditions as well as for separate examination of the electron transfer from Fc* to D1-OTF reveal there are two important reaction pathways operating. One is a rate-determining second reduction of D1-OTF, thus electron transfer from Fc* to a mixed-valent intermediate [(CuCuI)-Cu-II(LO)](2+) (D2), which leads to [Cu-2(I)(LO)](+) that is coupled with O-2 binding to produce [Cu-2(II)(LO)(OO)](+) (D). The other involves direct reaction of O-2 with the mixed-valent compound D2 followed by rapid Fc* reduction of a putative superoxo-dicopper(II) species thus formed, producing D.