Enthalpic and entropic contributions to substituent effects on the ionization of meta- and para-methylbenzoic acids in EtOH-H2O mixtures have been examined in terms of enthalpic and entropic constants (rho(H), sigma(H) and rho(S), sigma(S)). The variations of these constants with the molar fractions of the mixed solvents are attributed to the solute-solvent interactions and the structural alteration of the mixed solvents.