The lipase-catalyzed ring-opening polymerization of ecaprolactone (e-CL) and its derivatives was revisited using seven commercial enzymes. Lipases from Pseudomonas fluorescens (AK) and porcine pancreatic lipase (PPL) gave the best results, in both reaction conversion and degree of polymerization. Dependency on temperature and added concentration of enzyme was investigated, and there was a linear correlation between M n and the conversion ratio. The reaction proceeded rather slowly and the residual activity of these enzymes after prolonged incubation in e-CL was studied. There was a negative correlation between the conversion ratio in the polymerization reaction and the tolerance of the enzymes for the solvent (monomer). Accordingly, a mechanism involving enzymatic ring-opening and non-enzymatic (but catalytic) polymerization was proposed.