The thermo-oxidative degradation behavior of high-density polyethylene (HDPE) was investigated during oven aging at 100 degrees C. (1)H low-field solid-state nuclear magnetic resonance (NMR) was used to characterize the behaviors of molecular chains and the changes in phase content of HDPE during aging. NMR free induction decays (FIDs) were fitted by three components corresponding to the crystalline phase, interphase, and amorphous phase, respectively. The prolongation of aging led to a progressive increase in the crystalline phase at the expense of the other two phases. A slight decrease in chain mobility of the crystalline phase and interphase was observed simultaneously. The results obtained from other traditional technical approaches are also discussed in the context of the molecular dynamics properties revealed by NMR. The reduction in molecular weight, in chain mobility and the increase in crystallinity during thermo-oxidation were the main factors, which caused the loss of mechanical performance of HDPE. POLYM. ENG. SCI., 51:2171-2177, 2011. (C) 2011 Society of Plastics Engineers