A modified CNDO-UHF procedure is used to discuss cobalt-oxygen interactions in CoO and [CoO2]q (q = -1, 0, +1, +2, +3). The cobalt orbitals that contribute most to the bonding in these systems are the 4s and 3d orbitals with relatively little contribution from the 4p orbitals. The ground state of CoO is calculated to be a2π with a π hole localized in the 3d orbitals. The dioxygen activation in the CoO2q systems is described in terms of the local electron configuration of the O2(π) antibonding orbitals together with the charge, Mulliken bond order and bond length of coordinated O2. The distortion properties are described in terms of the thermodynamic stability difference and the energy barrier for the peroxo to superoxo transformation. The CoO2 systems studied were classified into two groups depending on the degree of dioxygen activation and distortion properties. The lowest activation energy calculated to break the O-O bond corresponded to 69 kcal mol-1, suggesting that dissociative chemisorption of O1 on a single Co atom is not a likely process.