A cobalt chlorin complex (Co-II(Ch)) efficiently and selectively catalyzed two-electron reduction of dioxygen (O-2) by one-electron reductants (ferrocene derivatives) to produce hydrogen peroxide (H2O2) in the presence of perchloric acid (HClO4) in benzonitrile (PhCN) at 298 K. The catalytic reactivity of Co-II(Ch) was much higher than that of a cobalt porphyrin complex (Co-II(OEP), OEP2- = octaethylporphyrin dianion), which is a typical potphyrinoid complex The two-electron reduction of O-2 by 1,1'-dibromoferrocene (Br(2)Fc) was catalyzed by Co-II(Ch), whereas virtually no reduction of O-2 occurred with Co-II(OEP). In addition, Co-II(Ch) is more stable than Co-II(OEP), where the catalytic turnover number (TON) of the two-electron reduction of O-2 catalyzed by Co-II(Ch) exceeded 30000. The detailed kinetic studies have revealed that the rate-determining step in the catalytic cycle is the proton-coupled electron transfer reduction of O-2 with the protonated Co-II(Ch) ([Co-II(ChH)](+)) that is produced by facile electron-transfer reduction of [Co-III(ChH)](2+) by ferrocene derivative in the presence of HClO4. The one-electron-reduction potential of [Co-III(Ch)](+) was positively shifted from 0.37 V (vs SCE) to 0.48 V by the addition of HClO4 due to the protonation of [Co-III(Ch)](+). Such a positive shift of [Co-III(Ch)](+) by protonation resulted in enhancement of the catalytic reactivity of [Co-III(ChH)](2+) for the two-electron reduction of O-2 with a lower overpotential as compared with that of [Co-III(OEP)](+).