Crosslinked linear polyethylene/branched polyethylene blends have been studied by thermooxidation at different temperatures (between 60°C and 110°C), which are either below or above the melting temperature of each polyethylene in the blend. The modifications of the chemical structure of both polyethylenes and the blends were monitored by infrared spectroscopy during thermo-oxidation. The same oxidation products could be identified between the linear polyethylene and the branched polyethylene. Chemical derivatization treatments allowed confirming the thermo-oxidation mechanism of polyethylene and the same oxidation products for both polyethylenes. By using an Arrhenius approach, it has been shown that accelerated ageing above 80°C is not representative of thermal ageing below this temperature. Moreover, it was shown that there was no influence of the physical state of the polymer during thermal ageing on the observed non-Arrhenius behavior. Thus, extrapolating accelerated ageing tests to lower temperatures by an Arrhenius approach in order to predict the lifetime of the polymer leads to critical errors.