Poly(butylene terephthalate-co-diethylene terephthalate) random copolymers of various compositions acid molecular weights were synthesized in bulk and characterized in terms of chemical structure and thermal and rheological properties. All copolymers are partially crystalline and thermally stable up to about 300 degrees C. The main effect of copolymerization is a decrease in melting and glass transition temperatures with respect to PET homopolymer. The fusion temperatures are well correlated to composition by Baur＇s equation and the T(m)degrees and Delta H(m)degrees extrapolated values for PET are in good agreement with those reported elsewhere. The presence of diethylene terephthalate units was found to influence slightly the rheological behaviour in the melt. (C) 2000 Elsevier Science Ltd. All rights reserved.