The scope of this study consists in studying the effects of processing type on thermal stability of poly(ethylene terephthalate) (PET) and its nanocomposites prepared with organically modified clays. To achieve this goal, an intercalating agent was synthesized and montmorillonite type of clay modified with this intercalating agent was mixed with the PET by using melt extrusion and high-shear thermokinetic mixing method. According to the results, manganese in the raw claythough chemically boundwas found to be responsible for the decreased intrinsic viscosity (IV) values, i.e. decreased molecular weight in PET/organoclay nanocomposites. Besides, it was revealed that working on the thermokinetic mixer provided substantial contributions such as shorter processing times in comparison to the melt extrusion method, elimination of drying step before melt processing, which has been accepted as an inevitable process for PET so far, less thermal degradation because of short processing times, and more homogeneous and better dispersion of the clay particles in PET matrix phase. POLYM. ENG. SCI., 2012. (C) 2012 Society of Plastics Engineers