The effect of gamma-irradiation and temperature to accelerate, in a representative way, the ageing of EVA (Ethylene Vinyl Acetate)/EPDM (Ethylene Propylene Diene Monomer) blends was investigated. These blends constitute model materials of cable insulation used in Nuclear Power Plants (NPPs). Physicochemical evolutions of the blends were monitored by InfraRed (IR) spectroscopy, gel fraction measurements and Differential Scanning Calorimetry (DSC). After having pointed out the main similarities between the thermooxidative degradation (100 °C) and the radiooxidative degradation at room temperature, both stresses were combined to accelerate the ageing. It is shown that combining gamma-irradiation and high temperature, or subsequent thermooxidation after radiooxidation, accelerates the ageing but may cause distortion within the degradation mechanisms. These limitations of representativeness encountered are particularly due to an increase in chain scission reactions and formation of new crystallites in the blend.