The thermal stability and degradation mechanisms of three semicrystalline polymers (polyethylene terephthalate [PET], high-density polyethylene [HDPE], and polyamide 6 [nylon 6]) were studied. Thermogravimetric traces were acquired first at heating rates of 1 degrees C/min and 10 degrees C/min, and it was determined that the heating rate significantly affected the thermal decay curves of the three polymers. The results allowed the selection of specific temperatures at which to carry out heating and cooling cycles from room temperature to the molten state. The thermal behaviors of HDPE, nylon 6, and PET each had particular characteristics. HDPE showed the highest thermal resistance, whereas nylon 6 displayed the lowest. PET had the lowest activation energy for degradation, 93.5 kJ/mol, and retained 14 wt% after thermal recycling with no influence of molecular weight. Thermal cycling also revealed gradual morphological changes in HDPE, nylon 6, and PET, and their crystals changed from regular to branched spherulites with variations in the nucleation patterns. Fourier-transform infrared spectroscopy measurements allowed us to explain the early stages of degradation for each polymer. POLYM. ENG. SCI., 2019. (c) 2019 Society of Plastics Engineers