Biodegradations of random copolymers of (R)-3-methyl-4-oxa-6-hexanolide [(R)-MOHEL] with various lactones such as epsilon-caprolactone (24/76 ratio), delta-valerolactone (22/78), and beta-propiolactone (31/69) were examined with normal or acclimated activated sludge. As a consequence, the biodegradability was found to increase in the order poly[(R)-MOHEL-ran-epsilon-caprolaetone] > poly[(R)-MOHEL-ran-delta-valerolactone] > poly[(R)-MOKEL-ran-beta-propiolactone]. The mode of biodegradation of (R)-MOHEL/lactone copolymers with enzymes (cholesterol esterase etc.) resembles that of biodegradation with activated sludge. All the copolymers degraded better than the homopoly[(R)-MOHEL] or homopoly(lactone)s. In contrast, these latter polymers are inert to hydrolytic degradation by basic or acidic buffer solution. The major biodegradation product of poly[(R)-MOHEL-ran-epsilon-caprolaetone] (15/85) by enzymes is 6-hydroxycaproic acid as revealed by NMR spectroscopy and mass spectrometry, while the biodegradation product from poly[(R)-MOHEL-ran-delta-valerolactone] (18/82) is a linear delta-valeric acid dimer.