Employing C-13 CP/MAS NMR spectroscopy, the existence of immobile regions in natural rubber (cis-1,4-polyisoprene) corresponding to a few percent of the monomer units has been detected at ambient temperature. For synthetic rubbers no immobile regions have been detected at all. Applying different physical and chemical treatments to natural rubber it is shown that mastication, gamma-irradiation, and increasing the temperature, slightly above the ambient, reduce the amount of immobile regions.