The relative importance of the rate of hydrolysis reaction and the rate of water diffusion in the degradation of polyglycolide (PGA) has been investigated. Samples were degraded in standard phosphate-buffered saline, and in fully and partially deuterated buffer solutions. Although the diffusion rate of deuteratcd water in PGA is similar to that of water, the acid-catalysed hydrolysis of the ester is significantly slower in D2O because of the kinetic isotope effect. The degradation process was examined using small angle X-ray scattering, mass loss and water uptake measurements, pH measurements and drug release profiles. The results show that the hydrolysis rate has an important effect and that the system is not wholly diffusion controlled. The results are consistent with a four-stage reaction-erosion front model of degradation. (C) 2003 Elsevier Science Ltd. All rights reserved.