Poly(butylene adipate-co-succinate) (PBAS), an aliphatic polyester, is known for its excellent biodegradability, but its physical and mechanical properties are poor. To improve the physical properties, stiff aromatic rings were added to PBAS through transesterification with poly(ethylene terephthalate) (PET). New biodegradable copolyesters were prepared by the intermolecular ester-exchange reactions between molten PBAS and PET. The transesterification reaction was carried out at 280 degreesC without a catalyst. The newly synthesized copolyesters were characterized with H-1-NMR spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. The mechanical properties were measured with a universal test machine, and the biodegradability was also investigated. By the new peaks appearing in H-1-NMR spectra of the copolyesters, the occurrence of the transesterification reaction between PBAS and PET was confirmed. A reduction of the melting temperature was observed for the copolyesters. The elongations at break of the new copolyesters increased for all compositions and reaction times, in comparison with PBAS. However, the tensile strength decreased with the induction of terephthalate units in the copolyesters. The biodegradability of the copolyesters also depended on the number of terephthalate units. (C) 2004 Wiley Periodicals, Inc.