Poly(ethylene terephthalate) (PET) was synthesized by self-condensation of bis-(a-hydroxyethyl) terephthalate (BHET). Copolymerization of BHET with ethyl, bis-3,5-(2-hydroxyethoxy) benzoate(EBHEB) and ethyl, 3-(2-hydroxyethoxy) benzoate (E3HEB) yielded copolymers that contain varying amounts of branching and kinks, respectively. Copolymers of BHET with ethyl, 4-(2-hydroxyethoxy) benzoate (E4HEB), in which only the backbone symmetry is broken but without disruption of the linearity, were also prepared for comparison. The composition of the copolymers were established from their H-1-NMR spectra. The intrinsic viscosity of all the copolymers indicated that they were of reasonably high molecular weights. The thermal analysis of the copolymers using DSC showed that both the melting temperatures (T-m,) and the percent crystallinity ( as seen from the enthalpies of melting (Delta H-m,) decreased with increasing comonomer (defect concentration) content, although their glass transition temperatures (T-g,) were less affected. This effect was found to be most pronounced in the case of branching, while the effects of kinks and linear disruptions, on both T-m and Delta H-m,, were found to be similar.