The protonation constants log k(app) of a series of disubstituted benzoic acids in aqueous solution at different temperatures between 5 degrees and 55 degrees C have been determined potentiometrically. The data of log k(app) have been analyzed under the light of a statistical thermodynamic model. The curvature of the function log k(app) = f (1/T) is related to the number n(w) of water molecules involved in the protonation and hydration reaction. The upward concavity of the curves of dinitro compounds are steeper that those for monosubstituted acids and imply higher number of water molecules. The curves of polyalkyl-substituted benzoic acids as determined by other authors show opposite (downward concavity) curvatures corresponding to negative numbers n(w) of water molecules. The values of log k(app) at 25 degrees C of disubstituted and polyalkyl-substituted benzoic acids plotted against the Hammett substituent constants sigma(Ham) deviate significantly from the line of the Hammett model.