Glycolide (GL) and epsilon-caprolactone (CL) were copolymerized in bulk at relatively high temperatures using stannous octoate as a catalyst. To investigate the relationship among microstructure, thermal properties, and crystallinity, three series of copolymers prepared at various reaction temperatures, times, and comonomer feed ratios were prepared and characterized by H-1 and C-13 NMR, DSC, and wide-angle X-ray diffraction (WAXD). The 600-MHz H-1 NMR spectra provided information about not only the copolymer compositions but also about the chain microstructure. The reactivity ratios (r(G) and r(C)) were calculated from the monomer sequences and were 6.84 and 0.13, respectively. In terms of overall feed compositions, the sequence lengths of the glycolyl units calculated from the reactivity ratios exceeded those measured from the polymeric products. Mechanistic considerations based on reactivity ratios, monomer consumption data, and average sequence lengths are discussed. The unusual phase diagram of GL/CL copolymers implies that the copolymer melting temperature does not depend on its composition alone but rather on the nature of the sequence distribution. The DSC and WAXD) measurements show a close relationship between polymer crystallinity and the nature of the polymer sequence.