This article is devoted to the study of electron-beam-induced degradation under argon atmosphere of an ethylene-propylene-diene monomer (EPDM, based on 5-ethylidene 2-norbornene) and an ethylene-propylene rubber (EPR) containing the same molar ratio of ethylene/propylene. The chemical structure modifications of polymeric samples were analyzed by ultraviolet-visible and IR spectroscopies. Crosslinking reactions were deduced by measuring the changes in gel fraction and the degree of swelling in n-heptane. Irradiation of EPDM and EPR created trans-vinylene, vinyl, vinylidene, and dienic-type unsaturations. The radiochemical yields for unsaturation formations in EPDM and EPR were similar. Degradation also involved crosslinking and the production of molecular hydrogen. The comparison between EPDM and EPR showed that the diene (in which a double bond is consumed with a high radiochemical yield) contributes to the increase in rate and intermolecular bridges density. Mechanisms are proposed to account for the main routes of EPDM degradation. (C) 2004 Wiley Periodicals, Inc.