The catalytic activities of cutinase immobilized on zeolite NaY and Candida antarctica lipase B immobilized on an acrylic resin (Novozym 453) were measured in a model transesterification reaction in three imidazolium cation-based ionic liquids (RTILs), supercritical ethane (sc-ethane), SC-CO 2 and n-hexane, at a water activity (a w) of 0.2 and 0.7. The transesterification activity of cutinase was highest and similar in l-n-butyl-3-methylimidazolium hexafluorophosphate ([C 4mim][PF 6]), sc-ethane and n-hexane, more than one order of magnitude lower in SC-CO 2, and increased with an increase in a w. Hydrolysis was not detected in sc-fluids and /7-hexane, and was observed in RTILs at a w 0.7 only. Both initial rates of transesterification and of hydrolysis of Novozym decreased with an increase in a w. Sc-CO 2 did not have a deleterious effect on Novozym activity, which was as high as in sc-ethane and n-hexane. The low reaction rates obtained in this case in RTILs suggested the existence of internal diffusion limitations absent in the cutinase preparation where the enzyme is only adsorbed at the surface of the support. sc-CO 2 did not adversely affect the catalytic activity of cutinase suspended in [C 4mim] [PF 6], suggesting a protective effect of the RTIL. In the case of Novozym, a marked increase in the rate of transesterification was obtained in the [C 4mim][PF 6]/sc-CO 2 system, compared to the RTIL alone. This may reflect improved mass transfer of solutes to the pores of the immobilization matrix due to a high concentration of dissolved CO 2 and a reduction in viscosity of the RTIL.