The inhibition activity of chlorophyll a derivatives with different substituents on the macrocycle and their Zn(II) and Co(II) chelates has been studied in the model reaction of ethylbenzene oxidation. It has been found that the values of the induction time (tau) and of the initial oxidation rate in the presence of the additives depend on the substituent on the macrocycle and the central metal atom. The inhibition parameter f (tau) k (7) (f (tau) is the stoichiometric inhibition factor and k (7) is the rate constant of antioxidants with peroxy radicals) has been measured; its value is increased by more than an order of magnitude by introducing the metal atom into the macrocycle. The values of f (tau) = 1.3-2.5 calculated from the induction time and f (c) a parts per thousand 1 calculated from the rate of the consumption of the Zn(II) and Co(II) complexes show that the conversion products of the complexes possess inhibition activity. The values of f (tau) and f (tau) k (7) for the Co(II) complexes with the amide group in the ligand environment depend on their initial concentration, a behavior that may be due to the reaction of the complexes with the hydroperoxide.