The CoIIcomplexes, Co(II)-bissalicylal-3,3′-diimino-di-n-propylamine peroxide I and decaammine-μ-peroxodicobalt cation II, were investigated by the electron spin resonance (esr). On the basis of the observed esr parameters, it was concluded that the unpaired electron is largely localized on the O2group and, therefore, these complexes are better described as super-oxides. The equilibrium in solution of I between the paramagnetic and dia-magnetic forms implied by the previous studies was also demonstrated.