A set of nickel(III) peroxo complexes bearing tetraazamacrocyclic ligands, [Ni-III(TBDAP)(O-2)](+) (TBDAP = N,Ni-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane) and [Ni-III(CHDAP)(O-2)](+) (CHDAP = N,N'-dicyclohexyl-2,11-diaza[3.3](2,6)pyridinophane), were prepared by reacting [Ni-II(TBDAP)(NO3)(H2O)](+) and [Ni-II(CHDAP)(NO3)](+), respectively, with H2O2 in the presence of triethylamine. The mononuclear nickel(III) peroxo complexes were fully chatacterized by various physicochemical methods, such as UV-vis, electrospray ionization mass spectrometry, resonance Raman, electron paramagnetic resonance, and X-ray analysis. The spectroscopic and structural characterization clearly shows that the NiO2 cores are almost identical where the peroxo ligand is bound in a side-on fashion. properties of the supporting ligands were confirmed by X-ray crystallography, where the CHDAP ligand gives enough space around the Ni core compared to the TBDAP ligand. The nickel(III) peroxo complexes showed reactivity in the oxidation of aldehydes. In the aldehyde deformylation reaction, the nucleophilic reactivity of the nickel(III) peroxo, complexes was highly dependent on the steric properties of the macrocyclic ligands, with. a reactivity order of [Ni-III(TBDAP)(O-2)](+) < [Ni-III(CHDAP)(O-2)](+). This result provides fundamental insight into the mechanism of the structure (steric) reactivity relationship of metal peroxo intermediates.