Two-step investigation was made to determine antioxidant properties of fullerenes C60/C70 and C70 in polystyrene. The antioxidative activity of fullerenes C60/C70 has been studied by model reaction of the initiated oxidation of styrene and then in accelerated tests of C60/C70 and C70 mixtures with polystyrene. It was established that the initiation and oxidation rates of the model reaction is substantially reduced in the presence of C60/C70. The rate constant for the addition of styryl radicals to C60/C70 was determined as K(333 K) = (9.0 ± 1.5).107 M-1 s-1. By differential scanning calorimetry (DSC) and thermogravimetry (TGA) it was demonstrated that fullerenes show a stabilizing effect comparable with the influence of the sterically-hindered phenol Irganox 1010 and amine Agerite White. The suggestion was made that the retarding effect of fullerenes is connected with its interaction with macroradicals R. leading to formation of less active compounds.