The lifetime of HV capacitors is drastically enhanced when epoxides are added to the liquid impregnant of the polypropylene films. The aim of this work was to understand the mechanism of action of such an additive. Aging of the dielectric was performed at a high AC field [135 MV/m]. The liquid impregnant, pure benzyltoluene or benzyltoluene with specially selected additives, was saturated with O-2 in order to accelerate PP aging. The degradation of the polypropylene films has been evaluated by the variation of their mean breakdown voltage and by FTIR microspectroscopy. The liquid impregnant has been characterized by gas chromatography. Our results show that the electrical stress enhances an oxidative degradation mechanism of the polymer and the liquid. Epoxides with two or more epoxy groups per molecule and other monomers of two or three functionalities inhibit this degradation. In the presence of such additives, electrical aging leads to the formation of a crosslinked polymeric deposit on the electrodes and the films. We attribute the good behavior of these components, and especially of epoxides in HV capacitors, to their ability to polymerize under an electrical stress and to form a protective layer that further prevents degradation.