Equilibrium measurements have been performed on the alpha-chymotrypsin-catalyzed hydrolysis reaction of N-acetyl-L-phenylalanine ethyl ester to (N-acetyl-L-phenylalanine + ethanol) with carbon tetrachloride, dichloromethane, toluene, and aqueous phosphate buffer as solvents for the reactants and products. Apparent equilibrium constants were measured as a function of temperature for this reaction in all four solvents. Calorimetric measurements were also performed for this reaction in aqueous phosphate buffer. The principal reaction occurring in the aqueous phosphate buffer at pH 6-7 is N-acetyl-L-phenylalanine ethyl ester(aq) + H2O(aq) N-acetyl-L-phenylalanine(-)(aq) + ethanol(aq) + H+(aq). Therefore, to compare the results for the reaction in water with those for the reaction in the organic solvents where it is assumed only neutral species are present, it was necessary to adjust the experimental results to the reaction involving neutral species : N-acetyl-L-phenylalanine ethyl ester(sln) + H2O(sln) = N-acetyl-L-phenylalanine(sln) + ethanol-(sln), where sln denotes either aqueous media, carbon tetrachloride, dichloromethane, or toluene. The values of the equilibrium constant for this latter reaction, with the concentration of water included in the expression for the equilibrium constant, ranged from 0.057 to 0.20 at T = 298.15 K for the four solvents. This rather limited range of values for the equilibrium constants is significant. The very limited amount of information available from the literature is also suggestive of the rule that equilibrium constants for hydrolysis reactions in different solvents are comparable if the reaction refers to neutral species and the concentration of water is included in the formulation of the equilibrium constant. Also, the standard molar enthalpy of reaction was found to be a linear function (slope = 313 K) of the standard molar entropy of reaction. This is indicative of an enthalpy-entropy compensation effect.