Hydrolysis of the novel tri-block copolymer poly(L-lactide-b-1,5-dioxepan-2-one-b-L-lactide) of different compositions has been studied in buffered salt solution at 37 degrees C and pH 7.4. Specifically, polymer weight loss, composition changes, molecular weight changes, thermal properties and release of lactic acid and 3-(2-hydroxyethyl)-propanoic acid have been detected. The degradation was found to start immediately after the sample was immersed into the aqueous buffer solution. The rate of degradation was influenced only by the original molecular weight, the copolymer composition had no significant effect. The heat of fusion and Tg increased with degradation time due to an increased amount of L-lactide (L-LA) in the polymer matrix. The GC-MS analysis showed that up to 70% of the theoretical amount of 3-(2 hydroxycthyl)-propanoic acid and 10-20% of lactic acid was released after 23 weeks of degradation.