Swollen state NMR spectroscopy is the most unequivocal estimate of crosslink densities in different phases. Signals of NMR spectra of polymers are broader than those of simple molecules and the signal width is increased as the polymer is crosslinked due to reduction in chain mobility. Swelling of the vulcanizates permits observation of spectra with sufficient resolution. Hydrogen nuclei of different elastomers that are in different environments are easily observed and are the most convenient tool