A series of poly(ethylene terephthalate) (PET) films with different additives were subjected to identical electrical stresses to investigate their partial discharge (PD) degradation behaviour. Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used to study the surface chemistry changes of the degraded samples. The filled samples showed markedly better PD resistance and lifetime compared to the unfilled PET. A filler ‘pile-up’ effect has been directly observed for the filled samples and is proposed as the mechanism underlying the enhanced stability. PD-induced breakdown results also revealed that TiO2 filled PET has a superior PD lifetime to BaSO4 filled PET, which could be attributed to both the higher permittivity of the TiO2 fillers and the voids that are created around the BaSO4 particles during the film orientation process. Further improvements to PD resistance and PD lifetime, through the reduction of surface oxidation, are observed for a BaSO4-filled sample additionally containing the UV stabilizer Tinuvin 1577.