The thermo-oxidation (70–160 °C) of two polymeric materials widely used as electrical insulators in cable systems was studied. The polymers are cross-linked polyethylene (XLPE) and cross-linked ethylene propylene rubber (XLEPR). Characterisation of the aged materials was performed by using FTIR spectroscopy, gel fraction, differential scanning calorimetry and tensile tests. Correlations were observed between the formation of oxidation products, the changes in crystallinity, the variation in gel content and the reduction in mechanical properties in both materials. By using a classical Arrhenius methodology, activation energies of XLPE and XLEPR were calculated in the temperature range 70–160 °C from spectroscopic measurements on 200 μm thick films and from tensile test results on 1 mm thick samples: the average values are 103 kJ mol−1 for XLPE and 102 kJ mol−1 for XLEPR. The results were consistent despite a diffusion-limited oxidation (DLO) effect observed in the 1 mm thick samples.