Cu(II) ion catalyzed kinetics of oxidation of H₂O₂ by [Ni^IIIL₂] (L₂ = 1,8-bis(2-hydroxyethyl)-1,3,6,8,10,13-hexaazacyclotetradecane) was studied in aqueous acidic medium in the presence of sulphate ion. The rate of oxidation of H₂O₂ by [Ni^IIIL₂] is faster than that by [Ni^IIIL₁] (L₁ = 1,4,8,11-tetraazacyclote-tradecane) in sulphate medium. DFT calculations at BP86/def2-TZVP level lead to different modes of bonding between [NiL]^II/III and water ligands (L = L₁ and L₂). In aqueous medium, two water molecules interact with [NiL]^II through weak hydrogen bonds with L and are tilted by $\sim$23° from the vertical axis forming the dihydrate [NiL]²⁺.2H₂O. However, there is coordinate bond formation between [NiL₁]^III and two water molecules in aqueous medium and an aqua and a sulphato ligand in sulphate medium leading to the octahedral complexes [NiL₁(H₂O)₂]³⁺ and [NiL₁(SO₄)(H₂O)]⁺. In the analogous [NiL₂]^III, the water molecules are bound by hydrogen bonds resulting in [NiL₂]³⁺.2H₂O and [NiL₂(SO₄)]⁺.H₂O. As the sulphato complex [NiL₂(SO₄)]⁺.H₂O is less stable than [NiL₁(SO₄)(H₂O)]⁺ in view of the weak H-bonding interactions in the former it can react faster. Thus the difference in the mode of bonding between Ni(III) and the water ligand can explain the rate of oxidation of H₂O₂ by [Ni^IIIL] complexes.