The chemical modifications occurring during the thermal degradation of sulfur-cured natural rubber were investigated with C-13-NMR. Samples of vulcanized rubbers were heat-aged at 70degreesC under an air (oxidative) environment for 3, 7, and 21 days. Changes were analyzed by the measurement of the variations of the concentrations of different C-S-x-C crosslinked structures during the thermooxidative process through the intensity of the related NMR lines in the 10-70-ppm range.