The Environmental Stress Cracking (ESC) of a poly(butylene terephthalate)/poly(butylene adipate) (PBT/PBA) copoly(ester ester) was studied by constant-load time-to-failure (TTF) tests performed on sharply notched tensile specimens in water and phosphoric acid solution at 353 K. For comparison, tests were also performed in air at the same temperature. It was found that the time to failure is significantly reduced both in water and in an acid solution as compared with air. ESC implies a mutual intensification of the destructive effect of loading and environment. Owing to the intrinsic hydrolytic instability of polyesters, a failure mechanism due to hydrolysis has to be excluded here to prove the ESC. Therefore the ESC test results are compared with measurements of the exclusive influence of hydrolysis on the degradation of mechanical properties. It was found that the PBT/PBA copoly(ester ester) tested is susceptible to environmental stress cracking in water and even more in a phosphoric acid solution, both at 353 K. From the comparison of the influence of water and a phosphoric acid solution on the time to failure, it was concluded that the ESC of PBT/PBA copoly(ester ester) in a phosphoric acid solution is mainly of a chemical nature. (C) 2004 Society of Plastics Engineers.