Poly(ethylene glycol terephthalate)-b-Poly (butylene terephthalate) copolymer (PELT-b-PBT) films with different copolymer compositions were incubated in phosphate buffered saline under pH 7.4 at 37 degrees C to study hydrolytic degradation and morphology up to 300 days. With the fall of intrinsic viscosity and mass of degraded films, SEM micrographs show that a set of particular and highly interconnected porous morphologies closely related to the content of PBT hard segments in copolymer is developed. Moreover, the variation in PBT crystallinity for copolymer films with weight ratio of 70/30 fluctuates with the development of degradation profiles, and PEGT content for copolymer films with weight ratio of 80/20 and 70/30 gradually decreases. The hydrolytic experiments demonstrate that the degradation of PELT-b-PBT copolymer results froth the cleavage of ester bonds between hydrophilic PEG and terephthalate. At the beginning period of degradation, PEGT-b-PBT copolymer films follow a typical mechanism of bulk degradation, and then undergo both bulk degradation and surface erosion, all of which finally generate the particular porous morphologies for copolymer films. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 1462-1470, 2009