Four-electron reduction of O₂ by octamethylferrocene (Me₈Fc) occurs efficiently with a dinuclear cobalt-μ-1,2-peroxo complex, 1, in the presence of trifluoroacetic acid in acetonitrile. Kinetic investigations of the overall catalytic reaction and each step in the catalytic cycle showed that proton-coupled electron transfer from Me₈Fc to 1 is the rate-determining step in the catalytic cycle.