The transesterification reaction of N-acetyl-L-phenylalanine ethyl ester with 1-propanol catalyzed by alpha -chymotrypsin was examined in the ionic liquids 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and 1-octyl-3-methylimidazolium hexafluorophosphate ([omim][PF6]), and in combination with supercritical carbon dioxide (SC-CO2). The activity of alpha -chymotrypsin was studied to determine whether trends in solvent polarity, water activity, and enzyme support properties, observed with this enzyme in conventional organic solvents, hold for the novel environment provided by ionic liquids. ct-Chymotrypsin freeze-dried with K2HPO4, KCI, or poly(ethylene glycol) demonstrated no activity in [bmim][PF6] or [omim][PF6] at very low water concentrations, but moderate transesterification rates were observed with the ionic liquids containing 0.25% water (v/v) and higher. However, the physical complexation of the enzyme with poly(ethylene glycol) or KCl did not substantially stimulate activity in the ionic liquids, unlike that observed in hexane or isooctane. Activities were considerably higher in [omim][PF6] than [bmim][PF6]. Added water was not necessary for enzyme activity when ionic liquids were combined with SC-CO2. These results indicate that [bmim][PF6] and [omim][PF6] provide a relatively polar environment, which can be modified with nonpolar SC-CO2 to optimize enzyme activity.