Copolymerizations of epsilon-caprolactone (epsilonCL) and glycolide (GL) were conducted in bulk at 120 degrees C with variation of the reaction time. Either Sn(II) 2-ethylhexanoate (SnOct(2)) or bismuth(III)subsalicylate (BiSS) were used as initiators combined with tetra(ethylene glycol) as co-initiator. The resulting copolyesters were analyzed by (1)H and (13)C NMR spectroscopy with regard to the total molar composition and to the sequence of the comonomers. Furthermore, two series of copolymerizations (either Sn- or Bi-initiated) were performed at constant time with variation of the temperature. It was found that BiSS favors alternating sequences more than SnOct(2). Time-conversion curves and MALDI-TOF mass spectrometry of homopolymerization suggest that SnOct(2) is the more efficient transesterification catalyst. A hypothetical reaction mechanism is discussed.