The thermal stability of poly(ethylene-co-trimethylene terephthalate) was first studied by thermogravimetry under nitrogen atmosphere at different heating rates. The results showed that the thermal behavior of the copolyester had a strong dependence on the chemical composition. The average activation energy from Ozawa technique increases with the concentration of EG units in the polyester, and a greater TG concentration in the copolyesters would decrease the onset degradation temperature. These phenomena may be attributed to the presence of one more methylene of TG unit than of EG in the copolymer. It is also noted that the yield of solid residue increases with the concentration of EG units in the polymer chain at any heating rate, which may be associated with the content of aromatic ring in the polymer chain. (c) 2006 Wiley Periodicals, Inc.