The changes of crystal structure and morphology in poly(glycolide), PGA, homopolymer and poly(glycolide-co-L-lactide), PGA-co-PLA, (90/10) random copolymer during in vitro degradation were investigated by gel permeation chromatography (GPC), wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS). GPC results showed that the molecular weight and polydispersity decreased significantly during the first 2 weeks of degradation. In contrast, the mass degree of crystallinity, phi(mc), determined from WAXD showed an Avrami-like behavior in both polymers, increasing rapidly within the first 2 weeks and gradually reaching a plateau value. The effect of degradation on the crystal unit cell dimensions was noticeable and anisotropic, which may reflect the process of crystal perfection in vitro. Corresponding SAXS results also showed that the long period (L), lamellar thickness (l(c)), and amorphous layer thickness (l(a)) from the crystal lamellar stacks all decreased appreciably in both PGA and PGA-co-PLA. samples during the first 3 weeks of degradation. By comparing molecular weight and lamellar thickness results, we conclude that the fully degraded chain fragments have an average length of about 3 times the crystal thickness. On the basis of these results, we propose that degradation proceeds through the combined processes of chain scission and cleavage-induced crystallization in the amorphous regions via two pathways. (1) The degradation occurs in the amorphous gaps between the crystal lamellar stacks, where the amorphous chains are broken leading to greater mobility to form new crystal lamellar stacks with thinner thickness. This process significantly reduces the averaged values of L, L-c, and l(a). (2) The degradation process also occurs in the amorphous layer domain between the adjacent lamellae within the lamellar stacks, where chain scission causes the rapid decrease in polydispersity.