By using thermal and NMR analyses with supporting evidence from X-ray and scanning electron and optical microscopy, this study has attempted to clarify confusing issues of physical miscibility vs. chemical trans-reaction in blends of aryl polyesters upon heating. The study demonstrated that the blends of poly(pentylene terephthalate) (PPT) with poly(ethylene naphthalate) (PEN) were initially immiscible; however, with heating/annealing at high temperatures (300 degrees C) for long enough times, the original two phases merged into one single phase composed of two polyesters and some minor fractions of copolyesters. Upon extended heating, however, two original polyesters disappeared, and a random copolyester, coded as EN-co-PT, of various sequence lengths was produced as a result of extensive trans-reactions between PEN and PPT. The trans-reacted products from heated PEN/PPT (50/50) blend were characterized using H-1 NMR. The sequence structures of the produced co-polyesters and intermediate products were determined by a triad analysis, which showed that the mean sequence lengths became shorter and the randomness increased with time of heating. X-ray analysis confirmed that the PEN/PPT (50/50) blend completely lost its crystallizability only when heated at 300 degrees C for time of 60 min or longer, indicating formation of fully random copolyesters. (c) 2005 Elsevier Ltd. All rights reserved.