The enantioselective esterification or ibuprofen (2-(4-iso-butylphenyl)propionic acid), catalyzed by Candida. antarctica lipase (type B), was performed in various organic solvents. The reactions were conducted in controlled water activity atmospheres, thereby permitting the influence of the solvents to be separated from their ability to strip water from the solid enzymes. Even in these constant water activity conditions, hydrophobic solvents allow higher enzyme activity than hydrophilic ones, indicating that hydrophilic solvents impede enzyme activity in essence and not because they strip water from the enzyme. The highest enantioselectivity was obtained in solvents leading to low reaction rates.Different properties were used to describe the solvents, namely the hydrophobicity, quantified by log P; epsilon, the dielectric constant : and E-T(N), a normalized electron acceptance index. None gave a clear and predictive portrait of the influence of the solvent, however, the hydrophobicity was the most satisfactory. This could be linked to the solubility of ibuprofen, which varies linearly with the value of log P.The highest enantioselectivity was observed at low water activities. This is caused by different effects of water activity on the reaction rates with the two isomers. The activity toward the less favored one decreased after reaching a maximum while the rate with the other isomer continued to rise when a(W) decreased.