Thermo-oxidation (70–160 °C) of peroxide cross-linked polyethylene (XLPE) formulations stabilised with different antioxidants were studied. In order to separately study the influence of three commonly used antioxidants, three formulations were prepared with only one antioxidant each (0.3% w/w): a sterically hindered phenolic antioxidant (Irganox 1010), an organosulfur antioxidant (Hostanox SE4) and an oligomeric, sterically hindered amine light stabiliser (HALS 94). The materials were characterised by infrared spectroscopy measurements and tensile testing. Results proved that the addition of antioxidants causes in each case an Arrhenius break, occurring at a different temperature depending on the nature of the added antioxidant. Interestingly, the study of the thermo-oxidation (50–105 °C) of a certified antioxidant-free polyethylene (non-cross-linked) revealed no Arrhenius deviation. In a previous paper, we reported an Arrhenius deviation at low temperature (50 °C) for two cross-linked PE and EPR, containing residual process antioxidants. These results strengthen our hypothesis that several Arrhenius deviations could be explained by the presence of antioxidants (residual process antioxidants or formulation antioxidants), which “activity” is extremely dependent on the temperature.