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The current paper aims to study the thermal stability of ethylene–propylene–diene monomer (EPDM) and styrene–ethylene–butylene–styrene (SEBS) block copolymer attained by radiation processing and evaluated by chemiluminescence (CL). Three blends with different weight ratios (1:3, 1:1 and 3:1), as well as individual rubbers, are γ-irradiated at 25, 50, 100 and 150 kGy. Their thermal stabilities are intercompared, and the activation energies required for oxidative degradation are calculated by using the values of oxidation induction times. Another investigation concerning the development of gel is in good agreement with the CL results. The aspects related to the mechanisms of the radiation fragmentation of blended components are discussed. The contributions of the blending components are evaluated based on the peculiar kinetic parameters, namely oxidation induction time (OIT) and onset oxidation temperatures (OOTs). It is clearly demonstrated that the EPDM component serves as the main source of radicals required for crosslinking, while the SEBS skeletons become the structural frames for the new crosslinked branches. The activation energies increase from 50 kJ mol−1 to 59 kJ mol−1 for unirradiated materials, but the increase for the blends exposed to 100 kGy is significantly larger from 41 kJ mol−1 to 54 KJ mol−1. The growth in the blending proportion of SEBS improves the thermal stability of the resulting materials. It is observed that the largest differences in the thermal resistances of γ-irradiated compounds are obtained for the samples exposed at 150 kGy, when the participation of each of the components is taken into account. This study highlights the research on and the productive methods of polymer processing, and the study of the irradiation of blends generates high-performance technical articles by the appropriate selection of technological parameters.