We have synthesized poly(octamethylene terephthalate) (POT), poly(octamethylene 2,6-naphthalate) (PON), and poly(octamethylene terephthalate-co-octamethylene 2,6-naphthalate)s [P(OT-co-ON)s] with various comonomer composition by melt-polycondensation reaction and investigated their chain structures, crystalline structures, melting and cocrystallization behavior by using H-1 NMR spectroscopy, wide angle X-ray diffraction (WAXD), and differential scanning calorimetry (DSC), respectively. It was observed that P(OT-co-ON)s exhibit clear melting and crystallization peaks in DSC thermograms and sharp diffraction peaks in WAXD patterns throughout the copolymer composition, resulting from the cocrystallization behavior of OT and ON units in copolymers, When the melting and crystallization temperatures of P(OT-co-ON)s are compared as a function of the copolymer composition, there exists an eutectic point at around 23 mol% ON, where the crystal transformation from POT-type to PON-type occurs. it was confirmed from WAXD patterns of the melt-crystallized samples that the crystal transformation from POT-type to PON alpha-type to PON beta-type occurs with the increment of the comonomer ON content in copolymers, i.e., POT-type crystals for POT and P(OT-co-ON) with 11 mol% ON, PON alpha-type crystals for P(OT-co-ON)s with 23-48 mol% ON, and PON beta-type crystals for PON and P(OT-co-ON)s with 62-87 mol% ON. Both DSC and WAXD results demonstrate the isodimorphic cocrystallization of P(OT-co-ON)s. Based on the Wendling-Suter model for cocrystallization thermodynamics, it was found that the average defect free energy for the inclusion of OT units into PON D-type crystals is much lower than the value of the incorporation of ON units into POT-type crystals. (C) 2009 Elsevier Ltd. All rights reserved.