AbstractEthylene–propylene–diene rubbers (EPDM) with 2‐ethylidene‐5‐norbornene (ENB), dicyclopentadiene (DCPD), and 1,4‐hexadiene (HD) as third monomers have been vulcanized with peroxide and with a conventional sulfur vulcanization recipe, and their devulcanization was subsequently investigated for recycling purposes. The behavior of these vulcanizates during pure thermal devulcanization depends on the EPDM third monomer and the crosslinker used. Peroxide vulcanizates of ENB‐EPDM devulcanize only to a small extent and predominantly by random scission, whereas peroxide vulcanizates of HD‐EPDM devulcanize by crosslink scission. In contrast, sulfur vulcanizates of ENB‐EPDM, devulcanize mainly by crosslink scission. During devulcanization of sulfur‐cured HD‐EPDM, scission of both crosslinks and main chains occurs. Sulfur‐cured DCPD‐EPDM cannot be devulcanized but shows further crosslinking instead. In those cases, where purely thermal devulcanization is already effective to a certain extent, diphenyldisulfide as devulcanization agent increases the effectivity during thermochemical devulcanization. Hexadecylamine as an alternative devulcanization agent is effective for ENB‐EPDM but does not contribute to thermochemical devulcanization of HD‐EPDM. In summary, devulcanization proceeds by different mechanisms in ENB‐EPDM, DCPD‐EPDM, and HD‐EPDM. Explanations are given in terms of the chemical structures of the third monomers, the corresponding crosslinks, and devulcanization agents. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008