Abstract The thermo-oxidative stability of poly(ethylene terephthalate) (PET) and low-density polyethylene (LDPE) films, coated with organic–inorganic hybrid coatings of various compositions, has been investigated after accelerated ageing tests, in order to ascertain a possible beneficial effect of these coatings on the electrical performances of these insulating materials. The results have shown that the coating affects degradation mechanisms for both LDPE and PET. Thermo-oxidation is slow in LDPE, leading to significantly better insulating characteristics after ageing: a strong reduction of the embrittlement time was also observed. The strong increase of crystallinity upon ageing, observed for both coated and uncoated PET samples, probably dominates the effect of ageing on the electrical properties. As a consequence, only slight beneficial effects on PET electrical performances have been observed (both on conductivity and electrical strength). On the other hand, the coating has a strong effect on molecular weight changes leading to a significant increase of molecular weight for coated PET, while uncoated PET undergoes a significant decrease. This suggests that coated PET should present significantly better properties upon very long times of ageing.