Through spectrophotometric and potentiometric measurements it has been verified that in aerated aqueous solutions of CoII containing tris(2-aminoethyl)amine (tren) as ligand, the doubly bridged μ-peroxo-μ-hydroxo dicobalt ion [Co2L2(O2)(OH)]3+ (L = tren) is totally formed at pH > 6.5. In aerated aqueous solutions of CoII and 3,7-diazanonane-1,9-diamine (dadn) at pH 6.5 only the singly bridged μ-peroxo dicobalt ion [Co2L2(O2)]4+ (L = dadn) is present. This complex at higher pH adds OH- ions in its axial positions; at pH > 10.5 the singly bridged μ-peroxo-dihydroxo dicobalt ion [Co2L2(O2)(OH)2]2+ is totally formed. The singly bridged complex [Co2L2(O2)]4+, after several hours, converts irreversibly into the doubly bridged μ-peroxo-μ-hydroxo ion [Co2L2(O2)(OH)]3+, while the single bridged [Co2L2(O2)(OH)2]2+ ion slowly autoxidizes to CoIII. By means of calorimetric measurements, usually carried out at constant pH, the enthalpies of formation of [Co2L2(O2)]4+ and [Co2L2(O2)(OH)2]2+ (L = dadn) and [Co2L2(O2)(OH)]3+ (L = tren) have been obtained. These results are compared with the enthalpies relative to the oxygen addition to [Co(trien)]2+ ion (trien = triethylenetetra-amine), and the enthalpy effects, connected with the change from a linear amine (trien) to its branched isomer (tren) and from the sequence 5,5,5 (trien) to the sequence 5,6,5 (dadn) of the chelate rings, are discussed.