Diblock copolymers consisting of methoxy poly(ethylene glycol) (MPEG) and poly(epsilon-caprolactone) (PCL), poly(delta-valerolactone) (PVL), poly(L-lactic acid) (PLLA), or poly(lactic-co-glycolic acid) (PLGA) as biodegradable polyesters were prepared to examine the phase transition of diblock copolymer solutions. MPEG-PCL and MPEG-PVL diblock copolymers and MPEG-PLLA and MPEG-PLGA diblock copolymers were synthesized by the ring-opening polymerization of epsilon-caprolactone or delta-valerolactone in the presence of HCl (.) Et2O as a monomer activator at room temperature and by the ring-opening polymerization of L-lactide or a mixture of L-lactide and glycolide in the presence of stannous octoate at 130 degreesC, respectively. The synthesized diblock copolymers were characterized with H-1 NMR, IR, and gel permeation chromatography. The phase transitions for diblock copolymer aqueous solutions of various concentrations were explored according to the temperature variation. The diblock copolymer solutions exhibited the phase transition from gel to sol with increasing temperature. As the polyester block length of the diblock copolymers increased, the gel-to-sol transition moved to a lower concentration region. The gel-to-sol transition showed a dependence on the length of the polyester block segment. According to X-ray diffraction and differential scanning calorimetry thermal studies, the gel-to-sol transition of the diblock copolymer solutions depended on their degrees of crystallinity because water could easily diffuse into amorphous polymers in comparison with polymers with a crystalline structure. The crystallinity markedly depended on both the distinct character and composition of the block segment. (C) 2004 Wiley Periodicals, Inc.