A set of aromatic-aliphatic copolyesters were synthesized by ring-opening polymerization of hexamethylene terephthalate (HT) cyclic oligomers and e-caprolactone monomer (CL) at 175 degrees C using Sb2O3 catalyst. A hexamer- and heptamer-enriched cyclic fraction isolated from the product obtained in the cyclo-depolymerization of poly(hexamethylene terephthalate) (PHT) was used. Ring-opening copolymerizations were conducted for HT to CL feed ratios covering the whole range of compositions and for increasing periods of reaction time. The comonomer contents of the resulting copolyesters were similar to those of their respective feeds, and did not vary with reaction time. In contrast, the copolyester microstructure evolved from block to random as the reaction progressed. The thermal properties of the copolyesters was apparently dependent on both composition and microstructure, and therefore changed with reaction time. The hydrolysis of the copolyesters in simulated physiological medium with and without addition of lipase was evaluated. Copolyesters containing more than 10% of CL were readily degraded in the presence of the enzyme. In contrast to PHT and other copolyesters with lower CL contents, the 50/50 copolyester also underwent extensive hydrolysis upon incubation at pH 10.2 and 80 degrees C.