Peroxidase-like activity of an azamacrocyclic Ni(II) complex, [Ni(II)(DMDAC)] 2+ (DMDAC = 1,8-dimethyl-1,3,6,8,10,13-hexaaza-cyclotetradecane = 1,8-dimethyl diazacyclam), was investigated by observing the oxidation of 2,2 0 -azinobis (3-ethylbenzothiazoline)-6-sulfonic acid (ABTS) in the presence of H 2 O 2 . UV/Vis and electron paramagnetic resonance spectral changes obtained from the mixture solution of [Ni(II)(DMDAC)](ClO 4 ) 2 , ABTS, and H 2 O 2 indicated that both ABTS and its oxidized form, ABTS +• , were the substrates of the peroxidase-like activity of [Ni(II)(DMDAC)] 2+ . Current work and previously reported experimental and theoretical works on the peroxidase-like activity of azamacrocyclic Ni(II) complexes suggest that these kind of macrocyclic Ni(II) complexes can generally exhibit peroxidase-like activity in aqueous solution and the catalytic cycle proceeds through the heterolytic cleavage of the O-OH bond in a Ni(II)hydroperoxo species.K E Y W O R D S azamacrocyclic Ni(II) complex, Ni-oxo complex, peroxidase activity