[2,8-C-13(2)] ENB was synthesized from labelled acetic acid and terpolymerized in a batch reactor to [2,8-C-13(2)] ENB-EPDM rubber (M-w = 4.8 x 10(5), d = 3, ENB content similar to 8 wt.%, ethene to propene ratio similar to 1.8). Labelled ENB, its precursors and the EPDM were characterized by NMR spectroscopy. ENB is incorporated in the polymer chain through the endo cyclic double bond. Upon high temperature exposure to air of an NMR sample, the number of double bonds decreases due to crosslinking via addition of backbone radicals to double bonds. Some oxidation of double bonds occurs, giving rise to acetyl norbornene units. No significant cleavage of double bonds takes place. The labelling/NMR approach provides an excellent tool for studying (minor) chemical conversions in EPDM rubber.