Proton-coupled electron-transfer reduction of dioxygen (O 2) to afford hydrogen peroxide (H 2O 2) was investigated by using ferrocene derivatives as reductants and saddle-distorted (α- octaphenylphthalocyaninato)cobalt(II) (Co II(Ph 8Pc)) as a catalyst under acidic conditions. The selective two-electron reduction of O 2 by dimethylferrocene (Me 2Fc) and decamethylferrocene (Me 10Fc) occurs to yield H 2O 2 and the corresponding ferrocenium ions (Me 2Fc + and Me 10Fc +, respectively). Mechanisms of the catalytic reduction of O 2 are discussed on the basis of detailed kinetics studies on the overall catalytic reactions as well as on each redox reaction in the catalytic cycle. The active species to react with O 2 in the catalytic reaction is switched from Co II(Ph 8Pc) to protonated Co I(Ph 8PcH), depending on the reducing ability of ferrocene derivatives employed. The protonation of Co II(Ph 8Pc) inhibits the direct reduction of O 2; however, the proton-coupled electron transfer from Me 10Fc to Co II(Ph 8Pc) and the protonated [Co II(Ph 8PcH)] + occurs to produce Co I(Ph 8PcH) and [Co I(Ph 8PcH 2)] +, respectively, which react immediately with O 2. The rate-determining step is a proton-coupled electron-transfer reduction of O 2 by Co II(Ph 8Pc) in the Co II(Ph 8Pc)-catalyzed cycle with Me 2Fc, whereas it is changed to the electron-transfer reduction of [Co II(Ph 8PcH)] + by Me 10Fc in the Co I(Ph 8PcH)- catalyzed cycle with Me 10Fc. A single crystal of monoprotonated [Co III(Ph 8Pc)] +, [Co IIICl 2(Ph 8PcH)], produced by the proton-coupled electron-transfer reduction of O 2 by Co II(Ph 8Pc) with HCl, was obtained, and the crystal structure was determined in comparison with that of Co II(Ph 8Pc).