Synergy was revealed in the action of binary mixtures of heavy metal chelates with N,N,N,N'-tetrasubstituted p-phenylenediamines in the initiated oxidation of model hydrocarbon ethylbenzene. The value of the synergistic effect (SE), defined as SE = (W-add/W-mix) - 1, where W-mix and W-add are the initial rates of oxygen uptake as obtained experimentally and calculated on the assumption of the additive action of the mixture components, respectively, increases with a decrease in the half-wave potential of the one-electron electrochemical oxidation E-1/2(ox) of both chelates and tertiary diamines.