Metal-dioxygen adducts are key intermediates detected in the catalytic cycles of dioxygen activation by metalloenzymes and biomimetic compounds. In this study, mononuclear cobalt(III)-peroxo complexes bearing tetraazamacrocyclic ligands, [Co(12-TMC)(O-2)](+) and [Co(13-TMC)(O-2)](+), were synthesized by reacting (Co(12-TMC)(CH3CN)](2+) and [Co(13-TMC)(CH3CN)](2+), respectively, with H2O2 in the presence of triethylamine. The mononuclear cobalt(III) peroxo intermediates were isolated and characterized by various spectroscopic techniques and X-ray crystallography, and the structural and spectroscopic characterization demonstrated unambiguously that the peroxo ligand is bound in a side-on eta(2) fashion. The O-O bond stretching frequency of [Co(12-TMC)(O-2)](+) and [Co(13-TMC)(O-2)](+) was determined to be 902 cm(-1) by resonance Raman spectroscopy. The structural properties of the CoO2 core in both complexes are nearly identical; the O-O bond distances of [Co(12-TMC)(O-2)](+) and [Co(13-TMC)(O-2)](+) were 1.4389(17) angstrom and 1.438(6) angstrom, respectively. The cobalt(III)-peroxo complexes showed reactivities in the oxidation of aldehydes and O-2-transfer reactions. In the aldehyde oxidation reactions, the nucleophilic reactivity of the cobalt peroxo complexes was significantly dependent on the ring size of the macrocyclic ligands, with the reactivity of [Co(13-TMC)(O-2)](+) > [Co(12-TMC)(O-2)](+). In the O-2-transfer reactions, the cobalt(III)-peroxo complexes transferred the bound peroxo group to a manganese(II) complex, affording the corresponding cobalt(II) and manganese(III)-peroxo complexes. The reactivity of the cobalt-peroxo complexes in O-2-transfer was also significantly dependent on the ring size of tetraazamacrocycles, and the reactivity order in the O-2-transfer reactions was the same as that observed in the aldehyde oxidation reactions.