A mononuclear cobalt(III)-peroxo complex bearing a macrocyclic tetradentate N4 ligand, [CoIII(TMC)(O2)]+ (TMC=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), was generated in the reaction of [CoII(TMC)]2+ and H2O2 in the presence of triethylamine in CH3CN. The reactivity of the cobalt(III)-peroxo complex was investigated in aldehyde deformylation with various aldehydes and compared with that of iron(III)- and manganese(III)-peroxo complexes, such as [FeIII(TMC)(O2)]+ and [MnIII(TMC)(O2)]+. In this reactivity comparison, the reactivities of metal-peroxo species were found to be in the order of [MnIII(TMC)(O2)]+ > [CoIII(TMC)(O2)]+ > [FeIII(TMC)(O2)]+. A positive Hammett rho value of 1.8, obtained in the reactions of [CoIII(TMC)(O2)]+ and para-substituted benzaldehydes, demonstrates that the aldehyde deformylation by the cobalt(III)-peroxo species occurs via a nucleophilic reaction.