Summary The antioxidant activity of selected N,N'-substituted p-phenylenediamines derived from 6PPD in polyisoprene matrix has been studied by differential scanning calorimetry (DSC) under non-isothermal conditions. The kinetic parameters describing temperature dependence of induction period have been obtained. Protection factors and antioxidant effectiveness have been calculated to characterize the stabilizing effect of the antioxidants under study. Using both criteria, the highest antioxidant activity has been observed in the case of Dusantox L, which is a mixture of 6PPD and its p-kumyl derivative. Its high antioxidant efficiency can be explained by the synergistic effect of 6PPD and its p-kumyl derivative. The lowest antioxidant efficiency of o-kumylderivative of 6PPD is probably caused by the sterical effect of the bulky kumyl group.