A complex aging regime takes place in the course of thermal aging of natural rubber. Depending on the kind and the content of the free crosslinker, the aging temperature, the duration of the aging period and the reactivity of the atmosphere (air or nitrogen) a different aging process can be observed also by NMR. The methods used are the common spin-echo-NMR (H-1) and parameterselective NMR-imaging (H-1). The decay of the echo-magnetization was evaluated on the basis of a modified single chain model (based on the tube model of de Gennes). The thermal aging course of a filled (carbon black) sample at 100 degrees C shows a characteristic behaviour, beginning with a strong decrease of T-2 followed by a weak increase through aging times of 10 to about 50 hours. At longer aging times the solid-like behaviour rises again.