Metal ion-coupled and proton-coupled electron-transfer reactions of dioxygen is described for one-electron, two-electron, and four-electron reduction of dioxygen by one-electron reductants such as ferrocene derivatives as well as by two-electron reductants Such as NADH analogs. The catalytic mechanism of the four-electron reduction of dioxygen with cofacial dicobalt porphyrins has been clarified based on the detailed kinetic Study and the detection of the intermediate.