Dioxomolybdenum complexes were examined as catalysts for the copolymerization of epsilon-caprolactone (epsilon-CL) and l-lactide (l-LA). The bis-[(5-OMe)salicylaldehydato]dioxomolybdenum complex completed the copolymerization after 20 h at 110 A degrees C with 0.05 mol% of the catalyst to produce a copolymer in high yield. The microstructure of the copolymer was analyzed using H-1 and C-13 NMR spectroscopy and was determined to have a random structure. The r values calculated from the heterodiad analysis of the H-1 NMR data were r (LA) = 0.91 and r (CL) = 0.93, and the L values calculated from the triad analysis of the C-13 NMR data were L (LA) = 1.58 and L (CL) = 1.81. Other dioxomolybdenum complexes, such as cis-alpha MoO2[(3-MeO)DiMeSaltn], MoO2(acac)(2) and (NH4)(8)[Mo10O34] exhibited comparable or slightly lower reactivity for the copolymerization. Consecutive polymerization of epsilon-CL followed by l-LA afforded a block copolymer without trans-esterification.