The selective two-electron reduction of dioxygen occurs in the case of a monocobalt porphyrin [Co(OEP)], whereas the selective four-electron reduction of dioxygen occurs in the case of a cofacial dicobalt porphyrin [Co-2(DPX)]. The other cofacial dicobalt porphyrins [Co-2(DPA), Co-2(DPB), and Co-2(DPD)] also catalyze the two-electron reduction of dioxygen, but the four-electron reduction is not as efficient as in the case of Co-2(DPX). The mu-superoxo species of cofacial dicobalt porphyrins were produced by the reactions of cofacial dicobalt(II) porphyrins with dioxygen in the presence of a bulky base and the subsequent one-electron oxidation of the resulting mu-peroxo species by iodine. The superhyperfine structure due to two equivalent cobalt nuclei was observed at room temperature in the ESR spectra of the mu-superoxo species. The superhyperfine coupling constant of the mu-superoxo species of Co-2(DPX) is the largest among those of cofacial dicobalt porphyrins. This indicates that the efficient catalysis by Co-2(DPX) for the four-electron reduction of dioxygen by Fe(C5H4Me)(2) results from the strong binding of the reduced oxygen with Co-2(DPX) which has a subtle distance between two cobalt nuclei for the oxygen binding. Mechanisms of the catalytic two-electron and four-electron reduction of dioxygen by ferrocene derivatives will be 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 turnover-determining step in the Co(OEP)-catalyzed two-electron reduction of dioxygen is an electron transfer from ferrocene derivatives to Co(OEP)I, whereas the turnover-determining step in the Co-2(DPX)-catalyzed four-electron reduction of dioxygen changes from the electron transfer to the O-O bond cleavage of the peroxo species Of Co-2(DPX), depending on the electron donor ability of ferrocene derivatives.