The ring-opening polymerization of mixtures of 2,2-dimethyltrimethylene carbonate (DTC) and L,L-lactide (LLA) in the melt; with diethyl zinc or dibutyltin dimethoxide as initiator, results in the formation of random copolymers. LLA polymerizes first in a fast reaction, followed by a slow polymerization of DTC. Investigations of the polymerization mechanism reveal that DTC is randomly inserted into LLA-LLA diads. The microstructure of the copolymers was determined by triad analysis of the C-13 NMR spectra. The thermal behavior of the copolymers, especially the dependence of the glass transition temperature on the-copolymer composition, was studied, and a deviation from the behavior predicted by the Fox equation was observed.