A low resolution solid state H-1 NMR T-2 relaxation time experiment is used in real time for the quantitative determination of the chemical crosslink density during accelerated sulphur vulcanization of gum-stock EPDM. The sampling rate, which was used in the present study, is about 30 seconds. It is shown that at 140 C the crosslink density increases fast during the first 30 minutes of the vulcanization and gradually increases even after 2 hours. A comparison with rheometer torque data suggests that at relatively low degrees of crosslinking trapping of transient chain entanglements provides a significant contribution to the increase of the rheometer torque. At high degrees of crosslinking the majority of transient entanglements has become trapped and the increase of the rheometer torque is just determined by the increase of chemical crosslink density. The NMR relaxation method has no special requirements to the samples used and determines the network density over the entire vulcanization process. The method may be extended for the analysis of filled and oil extended rubbers.