Four PET samples of different intrinsic viscosity values based on diverse manufacturing processes were characterized for their linear viscoelasticity using a parallel plate rheometer. Thermal stability and frequency sweep experiments were performed in the meh state at 270 degrees C. The influences of drying temperature, testing atmosphere (air or nitrogen), and preparation (granules vs. samples cut from compression molded sheets) were investigated. Results indicated that a higher drying temperature promoted complex viscosity significantly and compression molded sheets generated irreversible structural changes in harsh conditions. The samples measured in air decreased because of thermo-oxidation degradation, whereas that in nitrogen unexpectedly increased slightly because of polycondensation without surface renewal. The role of carboxyl end group towards PET thermal stability was found to be influenced by the atmosphere.