The transesterification between isoamyl acetate and ethanol in supercritical (SC) CO2, 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6], a room temperature ionic liquid), and in CO2 + [bmim][PF6] mixed solvent was studied at 65.0 degreesC and different pressures. p-Xylenesulfonic acid (p-TSA) was used as the catalyst. Results showed that the phase behavior of the reaction system affects the equilibrium conversion of the reaction in SC CO2 and CO2+[bmim][PF6] considerably. The mixtures of SC CO2 and ionic liquids, which are environmental benign solvents, may have potential applications in tuning the equilibrium conversions. (C) 2003 Elsevier B.V. All rights reserved.