Aliphatic polyesters containing thioester linkages were enzymatically prepared by both the copolymerization of lactone with mercaptoalkanoic acid and by the transesterification of polyesters with mercaptoalkanoic acids. The enzymatic copolymerization of epsilon-caprolactone with 11-mercaptoundecanoic acid (11MU) and 3-mercaptopropionic acid (3MP) was performed under reduced pressure using an immobilized lipase from Candida antarctica (CA). The transesterification of poly(epsilon-caprolactone) and poly[(R)-3-hydroxybutyrate] was carried out with 11MU and 3MP using lipase CA under reduced pressure.