Poly-DL-lactide-poly(ethylene glycol) (PELA) block copolymer containing 10% of polyethylene glycol (PEG) was prepared by bulk ring opening polymerization of lactide/PEG using stannous chloride as initiator. PELA microspheres containing human serum albumin (HSA) were elaborated by solvent extraction method based on the formation of double w/o/w emulsion. In vitro matrix degradation characteristics of these microspheres were performed in phosphate-buffered saline at pH 5.6, 7.4 and 9.6. The degradation profiles were characterized by observing the morphological change of microspheres and measuring the loss of microspheres mass, the decrease of polymer intrinsic viscosity, the decrease of pH value of degradation media, the reduction of polymer number-average molecular (M-n) and the change of polydispersity (M-w/M-n). We determined that the degradation rate of the microspheres matrix in the media was as follows: pH 9.6 > pH 5.6 > pH 7.4 during the initial 4 weeks and in the later incubation time the degradation rate was as follows: pH 5.6 > pH 7.4 > PH 9.6. It was suggested that the pH of the degradation media had a big effect on the degradation rate of HSA/PELA microspheres matrix. (C) 2002 Published by Elsevier Science B.V.