The effect of in vitro hydrolytic degradation on the microstructure of unoriented semicrystalline poly(glycolic acid) (PGA) was examined using simultaneous small-and wide-angle synchroton x-ray scattering (SAXS/WAXS) and ultraviolet (UV) spectrophotometry. Samples were degraded in buffer solutions at 37 degrees C and were examined wet to avoid the structural changes that occur on drying. During degradation, the crystal density remained constant, and little change was seen in the lateral extent of the crystal lamellae. The transition layer between the crystalline and amorphous phases roughened slightly. More dramatic changes were seen in the amorphous phase, resulting in sharp increases in the crystallinity, the amount of glycolic acid in the buffer solution, and in the density difference between the crystal lamellae and the layers separating them. These changes indicated a loss of amorphous material that leveled off after 30 days. The lamellar repeat distance fell from around 95 to 80 Angstrom in the first 20 days before slowly rising again towards its initial value, changes which are interpreted as reflecting a two-stage loss of amorphous material, in which highly coiled loops and tie chains are degraded faster than taut tie chains. Once the coiled material is removed, the taut chains are able to adopt entropically favorable conformations, pulling the crystals towards each other, lowering the lamellar repeat, and creating internal stresses within the spherulite. As these newly coiled chains degrade, the crystals are released and slowly separate. The changes in long period are also considered in the light of chemical changes during degradation. Such change of chemical environment could affect the affinity for water of the amorphous inter-lamellar regions and affect the swelling. The observed changes in the long period may be a consequence of either or both of these factors. These findings give microstructural information of importance in the prediction and control of mechanical properties during degradation and the diffusivity of other molecules through degrading PGA.