A kinetic model was proposed for enzymatic reactions in a reverse micellar system, involving a water-insoluble substrate, Though surfactant is one of the main structural components of reverse micelles, an increase in the surfactant concentrations affects the enzyme activity remarkably. A relationship between the enzyme activity and the surfactant concentration is discussed. In this study it was assumed that free substrate in the organic phase was in adsorption equilibrium with the surface of the micellar surfactant, and that the adsorption coefficient and the true K-m value (Michaelis constant) were independent of the surfactant concentration. The validity of this model was verified by data on the hydrolysis of olive oil, catalyzed by Chromobacterium viscosum lipase (Glycerolester hydrolase; EC 3.1.1.3) in an AOT/isooctane reverse micellar system. The activity value predicted by the model equation agreed well with the experimental data.