The conformational analysis of the mesomorphic phase structure in poly(ethylene terephthalate) (PET) films based on the determination of the amount of Ta (trans in the amorphous phase), Tc (trans in the crystalline phase), and G (gauche) isomers present in samples of PET films heat treated between 110°C and 230°C is presented. The change in the amount of isomers was related to the mesomorphic phase determined from a correlation among wide-angle X-ray diffraction (WAXD), Fourier transform infrared (FTIR), density, and sorption properties. The reduction in sorption in semicrystalline samples, with respect to completely amorphous samples, at low penetrant activity allowed determination of the fraction of impermeable phase. This fraction, when compared with crystallinity determined from WAXD and density techniques, is higher, indicating the presence of mesomorphic fractions that probably have a bidimensional order. Separation of the contribution of isomers Ta and Tc and its relation with the mesomorphic phase indicate that the mesomorphic phase in PET films is related to the amount of Ta isomer. Up to 180°C, the mesomorphic phase in PET films is composed predominantly of Ta isomer. Above 180°C, the Ta isomer tends to be transformed in Tc, and the difference in the values of the impermeable phase obtained by WAXD, density, and sorption suggests that there is a fraction of oriented noncrystalline phase that is stable and contributes to the mesomorphic phase. A thermal interchange diagram among Ta. G, and Tc isomer is proposed. Degradation aspects were considered in samples heat treated at temperatures higher than 180°C in O2 atmosphere.