The phase behavior and the morphology of PET-based copolyesters have been studied by DSC, optical and electron microscopy and WAXS. The materials were prepared by transesterification of PET with equilmolar amounts of sebacic acid (S) and hydroquinone (Q), and with varying amounts of 4-hydroxybenzoic acid (H). In no case a compositionally homogeneous random copolyester was obtained: even the synthesis carried out in the absence of H gave a product that, though appearing as a homogeneous liquid-crystalline copolymer when characterized by conventional calorimetric and microscopic techniques, was shown by solvent fractionation and X-ray analysis to contain a segregated phase, probably formed by aromatic-rich copolyester chains. An increase of the amount of added H and, thereby, of the overall degree of aromaticity of the mixture, was shown to favor the segregation phenomena. The results confirm that an enthalpy-driven compositional differentiation of the two phases of the investigated copolyesters takes place during synthesis.