The copolymers of ethylene carbonate (EC) with e-caprolactone (CL) or δ-valerolactone (VL) were synthesized using (C5Me5)2SmMe(THF) as initiator. We prepared EC/CL and EC/VL copolymers with a high molecular weight (Mn ≧; 140000) without decarboxylation from the EC unit. The EC/CL copolymers were obtained in a higher carbonate content (up to ca. 30 mol%) and in a better yield compared with EC/VL copolymers. The 1H NMR and thermal analyses revealed that these copolymers exhibit random arrangement. The enzymatic degradations of EC/lactone copolymers were investigated using lipoprotein lipase and cholesterol esterase (both from Pseudomonas sp.). The degradability of the copolymers was much improved compared with that of lactone homopolymers. Greater degradation was observed for the lipase-induced degradation than the cholesterol esterase. The biodegradability of these copolymers in natural environments (viz., in activated sludge and seawater) was examined. As compared with enzymatic degradations, similar good biodegradability was observed for EC/CL copolymers, while a poor degradation was seen for EC/VL copolymers.