A theoretical kinetic model has been developed in order to describe the enzyme reaction in organic solvents. In this model the hydration of the enzyme molecule was examined and the equilibrium kinetic constants expressed in terms of thermodynamic activity. Analysis of a proposed kinetic model shows that the enzyme reaction rate in organic solvents is determined by two factors : substrate solvation and enzyme hydration, which are determined by the activity coefficient of the substrate and the water activity of the reaction media, respectively. The activity coefficient of the substrate and the water activity have been calculated using the UNIFAC equation to analyze the effects of organic solvents on the rate of enzyme reaction, and the results were compared with experimental data. Predictions of the proposed model were found to be in good agreement with previous experimental observations.