The effects of water on enzyme (protein) hydration and catalytic efficiency of enzyme molecules in organic solvents have been analyzed in terms of the thermodynamic activity of water, which has been estimated by the NRTL or UNIFAC equations. When the amount of water bound to the enzyme was plotted as a function of water activity, the water adsorption isotherms obtained from the water-solvent liquid mixtures were similar to the reported water-vapor adsorption isotherms of proteins. The water adsorption of proteins from the organic media was not significantly dependent on the properties of the solvents or the nature of the proteins. It is also shown that there is a linear relationship between the logarithm of the enzyme reaction rate and water activity. However, the dependence of the enzyme reaction rate on water activity was found to be different depending on the properties of the solvent. The relationship between water activity and other solvent parameters such as solvent hydrophobicity and the solubility of water in the solvent is also discussed.