The reactivity toward dioxygen of two series of dicobalt cofacial diporphyrins in solution in an aprotic solvent is described. Some of these compounds are efficient electrocatalysts for the four-electron reduction of dioxygen when adsorbed on a graphite electrode immersed in aqueous acid. Their electrochemical and spectroscopic (UV-vis, EPR) behavior in solution shows that, contrary to what is observed with cobalt monomers, the neutral [PCoII (CoP)-P-II] (1) (P stands for a porphyrin ring) form does not react with dioxygen. Uniquely the one- and two-electron-oxidized forms of the dimer, [PCoII. (CoP)-P-II](+) (1(+)) and [PCoII-(CoP)-P-II](2+) (1(2+)), respectively, reversibly bind dioxygen, giving two complexes, 2 and 3, at room temperature and in the absence of a good axial ligand. The stability constants of the two O-2 complexes have been measured spectrophotometrically and/or electrochemically, and prove to be remarkably high. As a whole, the present O-2 binding processes appear unprecedented as basically different in many respects from the process classically described in the case of cobalt monomers. Extended Huckel molecular orbital (EHMO) calculations, based on the crystal structure of the Co(2)FTF4 dimer in its uncomplexed form (Go-Co distance 3.42 Angstrom), show that, in the absence of very important deformations of its structure, the only possible geometry for the O-2 complex of the two-electron-oxidized derivative [PCo-O-2-CoP](2+) (3) is the mu-eta(2):eta-(2)-peroxo structure. The calculated corresponding electronic diagram affords a rationale for most of the experimentally observed properties. Specifically, the O-2 complex of the one-electron-oxidized form [PCo-O-2(.)-CoP](+) (2), the reduced form of complex 3, should be considered as a species in which the O-2 moiety is further reduced, at least partially, as compared to its peroxo state in 3, i.e., consequently in an oxidation state intermediate between peroxo (-1) and oxo (-2). Preliminary results indicate that this Species reacts with one proton, while the two-electron-oxidized O-2 complex 3 is resistant to protonation. The possible implications of these specific properties of the dicobalt dimers in the four-electron reduction mechanism of O-2 are discussed, and structural and mechanistic similarities with bioinorganic dinuclear sites appear significant.