The synthesis of biodegradable polyesters for biomedical applications in supercritical dioxide has been the subject of a number of recent publications. However, systematic studies of the mechanism of polymerization in this novel medium are rare. In this work, we present the first systematic study of the ring-opening polymerization of (is an element of)-caprolactone catalyzed by the industrially important catalyst tin(II) ethyl hexanoate. We report on the effects of varying reagent concentrations and conditions on the products obtained after polymerization in ScCO2 and compare this with the results obtained in conventional reaction media.