The anionic ring-opening polymerization of epsilon-caprolactone in supercritical CO2 is described using different metal alkoxides as initiators. The solubility of the monomer in supercritical CO2 has been studied at various pressures and temperatures, leading to the determination of the corresponding phase diagrams. The study of the polymerization initiated by Y(OiPr)(3), La(OiPr)(3), and Al(OiPr)(3) has shown a difference in reactivity according to the metal and the temperature. The polymerization rate was also strongly decreased compared with that of polymerization carried out in regular solvents. In order to understand more fully this phenomenon, the interactions between initiators and CO2 have been characterized by IR spectroscopy. This has confirmed reactions between ionic alkoxides and CO2 to Yield carbonates, whereas less ionic species have a lesser reactivity toward CO2 (C) 2003 Elsevier B.V. All rights reserved.