Thermo-oxidation (50–160 °C) of two polymer materials widely used as electrical insulators in cable systems was studied: a peroxide cross-linked polyethylene (XLPE) and a peroxide cross-linked ethylene propylene diene monomer (XLEPR). The materials were characterised by infrared spectroscopy measurements and tensile testing. Arrhenius treatment was applied to the data, using the relative conversion rate methodology and the shift factor methodology over the entire temperature range. The physical state of the material (semi-crystalline or amorphous) was shown to have no influence on the Arrhenius behavior for two reasons: (a) no Arrhenius deviation was observed at the melting temperature of the polyethylene matrix (117 °C) and (b) an Arrhenius deviation was observed at the same temperature (50 °C) for the two polymers, despite their difference in terms of crystallinity and physical state at the ageing temperatures studied. This deviation at low temperature led to large overestimations of lifetime predictions for both materials (28% for XLPE and 78% for XLEPR).