Accelerated-sulfur-vulcanized C-13-labeled EPDM with and without carbon black and extender oil was analyzed using C-13 solid-state NMR to determine the chemical structure of the network. High-resolution solid-state C-13 NMR reveals that sulfur cross-linking takes place at the allylic positions of the ENB independent of the presence of carbon black and oil. From the integrated intensities of the C-13 signals, the conversion of ENB into a cross-link can be quantitatively determined during the vulcanization process. The ENB conversion for gumstock EPDM is similar to10% after 10 min of vulcanization at 150 degreesC, which is a typical optimum vulcanization time in commercial applications. In the presence of carbon black the ENB conversion is marginally faster and reaches similar to12% in 10 min, while a maximum conversion of similar to20% was obtained. The efficiency of the ENB conversion was similar to20% less at 150 degreesC and similar to30% less at 180 degreesC in the compound with carbon black and oil compared to the compound without carbon black and oil. The substitution at the 9-position of ENB is always preferred over each of the two 3-positions. In turn, the substitution on the 3-exo position is always preferred over the 3-endo position, which is different from earlier model studies. When the material is heated for extended periods (10-20 min), oxidation and reversion of the cross-links starts to occur. Reversion is enhanced upon a temperature increase to 180 degreesC and yields a 4,5,6,7-tetrahydro-4,7-methanobenzo[b]thiophene compound. The length of the sulfur bridge in compound A and B is rather short, i.e., I or 2.