The ring-opening polymerization of epsilon -caprolactone (CL) initiated by dibutyltin dimethoxide was studied in supercritical carbon dioxide at 40 degreesC, under a pressure of 210-215 bar. The polymerization is controlled as assessed by the linear dependence of M-n,M-SEC On conversion and of In([CL](0)/[CL]) on time. Moreover. there is a good agreement between M-n,M-SEC and M-n,M-calc, at least until 20 000 g/mol. The apparent rate constants of polymerization in different media increase as follows: sc CO2, CFC-113, toluene, bulk. The kinetic order in initiator for the polymerization in solution and in sc CO:! was extracted from the slope of In k(app) vs In [Sn](0). It appears to depend on the initiator concentration in solution, which is not the case in sc CO2.