Synergy was revealed in the influence of binary mixtures of various phenols with N,N,N'N'-tetramethyl-p-phenylenediamine (TMDA) on the inhibition of the initiated oxidation of ethylbenzene. The magnitude of the synergistic effect (SE) was calculated as SE = (W-mix/W-add) - 1, where W-mix and W-add are the initial rates of oxygen uptake as obtained experimentally and calculated on the assumption of additivity, respectively. Linear correlation between the SE value and the rate constant of antiradical activity was found for mixtures of unhindered phenols. Sterically hindered phenols drop out of this correlation series-the mixtures based on these are most effective. The synergy in the mixtures is presumably due to interaction between the components, which is indicated by the appearance of new absorption bands in optical spectra measured during ethylbenzene oxidation inhibited by antioxidant mixtures.