In this study, the effect of -rays on the mechanical and thermal properties of polymer composites based on chlorinated isobutylene–isoprene rubber/chlorosulphonated polyethylene rubber blend with varying contents of carbon black (CB) filler was investigated. The samples were irradiated at ambient conditions with 100, 200 and 400 kGy radiation doses. Tensile strength and hardness are increasing with CB content and radiation dose is increasing, while elongation at break is decreasing. Loss of mass of 0.5, 10 and 30% was calculated for the samples from their respective thermogravimetric curves. The thermal stability of the nanocomposites was improved by both the degree of loading with filler and the cross-linking induced by -irradiation.