Lipase-catalyzed synthesis reactions must be performed in nonaqueous media (organic solvents or solvent-free systems). The choice of the optimal solvent is usually, a fastidious task that necessitates the determination of kinetic parameters in each solvent. The approach used here, to overcome the lack of a model that can pr edict the kinetics whatever the solvent, consists in the use of thermodynamic activities instead of concentrations of components, and assumes that activity-based kinetic parameters are the same in all solvents. This assumption is discussed, and a solution is proposed which takes into account some observed residual solvent effects. The reaction chosen to test this approach was the esterification of oleic acid with ethanol catalyzed by an immobilized lipase, Lipozyme. For this reaction, the kinetics predicted in various organic solvents and in solvent-free systems is in agreement with the experimental data.