The kinetics of the alpha-chymotrypsin catalysed transesterification of N-acetyl-l-phenylalanine ethyl ester with 1-butanol and the competing hydrolysis were evaluated at fixed water activity in two ionic liquids and two non-ionic organic solvents. In most respects the four solvents behaved similarly. However, at a water activity of 0.33, higher catalytic activity was observed in the ionic liquid, 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide, than in the other solvents, and at a(w)=0.11 catalysis was only observed in this solvent.