The C-13 nuclear magnetic resonance (n.m.r.) spectra were found to provide a means for direct observation of the structures formed along the glycolide/lactide copolymer chain upon transesterification. Nevertheless H-1 n.m.r. spectroscopy appeared to be more convenient and useful in the analysis of chain microstructure. The glycolide methylene group region of the proton spectra exhibits several signals which can be ascribed to longer sequences (as compared to those giving rise to C-13 resonances) as well as to the sequences which can appear only as a result of the second mode of transesterification. The analysis revealed that the longest blocks were in the copolymers prepared with Al(acac)(3) as the initiator and the shortest blocks were in the copolymers obtained with tin initiators. A decrease in temperature was found to reduce the extent of the second mode of transesterification.