A cobalt chlorin complex (CoII(Ch)) efficiently and selectively catalyzed two-electron reduction of dioxygen (O2) by one-electron reductants (ferrocene derivatives) to produce hydrogen peroxide (H 2O2) 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 (CoII(OEP), OEP2- = octaethylporphyrin dianion), which is a typical porphyrinoid complex. The two-electron reduction of O2 by 1,1′-dibromoferrocene (Br2Fc) was catalyzed by Co II(Ch), whereas virtually no reduction of O2 occurred with CoII(OEP). In addition, CoII(Ch) is more stable than CoII(OEP), where the catalytic turnover number (TON) of the two-electron reduction of O2 catalyzed by CoII(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 O2 with the protonated CoII(Ch) ([CoII(ChH)]+) that is produced by facile electron-transfer reduction of [CoIII(ChH)]2+ by ferrocene derivative in the presence of HClO4. The one-electron-reduction potential of [CoIII(Ch)]+ was positively shifted from 0.37 V (vs SCE) to 0.48 V by the addition of HClO4 due to the protonation of [CoIII(Ch)]+. Such a positive shift of [Co III(Ch)]+ by protonation resulted in enhancement of the catalytic reactivity of [CoIII(ChH)]2+ for the two-electron reduction of O2 with a lower overpotential as compared with that of [CoIII(OEP)]+.