The kinetics of the oxygenation reaction of CoL22+ complexes (L = ethylenediamine (en), N,N'-dimethylethylenediamnine (dmen)) have been investigated in dimethyl sulfoxide (dmso) at 298 K and in a medium adjused to 0.1 mol dm(-3) with Et4NClO4 by means of a UV-vis spectrophotometric technique. The reaction mechanisms are consistent with the fast formation of superoxo 1:1 initial CoL2-O-2 species (L = en, dmen), whereas the dimeric mu-peroxo (CoL2)(2)O-2 adduct is formed only when L = en, in the rate determining step. The kinetic results are discussed taking into account the effects of ligand/solvent substitution. EPR results give information on the electronic structure and the coordination geometry of the Co(II) complexes and further confirm the stoichiometry of the species formed. Ab initio calculations provide insights on the geometrical parameters of all the complexes investigated and allow us to draw some hypotheses about the influence of H...H nonbonded interactions in the eventual formation of the dimeric mu-peroxo (CoL2)(2)O-2 complexes. Solvational effects are also considered. The formation of the (CoL3)(2)O-2 adduct is also proved when L = en by means of O-2 volumetric absorption.