The chemical changes that occur in thermal degradation of sulfur-cured natural rubber were investigated using C-13-NMR analysis. The crosslinked carbons formed during the vulcanization process appear in the NMR spectra as peaks representing the various structures formed. Samples of vulcanized rubber were heat-aged at 100 and 150 degrees C under both air (oxidative) and nitrogen (inert) environments to assess both temperature and oxidative effects. Changes were observed by measuring peak areas under the crosslinked carbon peaks. A- and B-type cis monolinkages were found to decompose much faster by oxidative factors at both 100 and 150 degrees C than under nitrogen at either temperature. The polylinkages, however, showed the opposite result of thermal degradation predominating over oxidative factors. Cis-trans isomerization of the main chain carbons was observed at the higher temperature while absent at 100 degrees C.