Detailed information about the chemical role of the third monomer 5-ethylidene-2-norbonene (ENB) in EPDM during peroxide cross-linking has been obtained by use of MAS C-13 NMR spectroscopy and H-1 NMR T-2 relaxometry. Advanced C-13 NMR techniques like INADEQUATE and TOCSY applied to a special EPDM grade with C-13-labeled ENB reveal a large number of new aliphatic and olefinic signals as well as indications for oxidation, The ENB unit is involved in cross-linking reactions not only (i) via addition of macroradicals to the pendent ENB unsaturation yielding aliphatic cross-link structures but also (ii) via combination of ENB-derived allyl radicals resulting in cross-link structures with intact unsaturation. The latter represents a novel pathway in the mechanism for peroxide curing of EPDM.