The syntheses and O-2 reactivities of activesite models of cobalt-substituted ring-cleaving dioxygenases are presented. The pentacoordinate cobalt(II)-aminophenolate complex, [Co(Tp(Me2))((tB)(u2)APH)], gives rise to two distinct dioxygen adducts at reduced temperatures. The first is a paramagnetic (S = 1/2) cobalt(III)-superoxo species that was characterized with spectroscopic and computational techniques. The identity of the second Co/O-2 adduct was elucidated by X-ray crystallography, which revealed an unprecedented cobalt-(III)-alkylperoxo structure generated by O-2 addition to the metal ion and ligand. These results provide synthetic precedents for proposed intermediates in the catalytic cycles of O-2-activating cobalt enzymes.