An ultrasonically-aided extrusion process was developed for fast in situ compatibilization of immiscible polymer blends. PET, PEN, and their 50/50 blend were ultrasonically extruded at various amplitudes. PET underwent homopolymerization and degradation, respectively, at ultrasonic amplitudes of 7.5 and 10 Am, while PEN underwent degradation at amplitudes of 5, 7.5, and 10 mu m. MALDI-TOF mass spectrometry revealed greater amounts of hydroxyl and carboxyl terminated oligomers in ultrasonically treated PET and PEN, indicating their greater reactivity. Ultrasonic treatment at short residence time led to the enhancement of transesterification reaction in the PEN/PET blend, as shown by H-1 NMR and MALDI-TOF, indicating greater randomization with ultrasonic treatment. The latter was also observed through a shift in T-g that closely follows Gibbs-DiMarzio relation and an increase in viscosity of blend with treatment at an amplitude of 10 Am. No crystallinity was observed in the blend due to the already high level of transesterification introduced by extrusion without treatment. Accordingly, crystallinity, mechanical properties, oxygen permeability, and optical clarity of the blend were not influenced by ultrasonic treatment. (C) 2010 Elsevier Ltd. All rights reserved.