On the basis of dissociation enthalpies from micro-calorimetric measurements by using LKB-2277 BioActivity Monitor, along with dissociation equilibrium constants, enthalpic and entropic contributions to substituent effects and solvent effects on the dissociation of acetic acid and three mono- substituted halogen-acetic acids (X-CH2COOH, X = H, Cl, Br, I) in binary mixtures of EtOH-H2O have been examined in terms of enthalpic and entropic reaction constants (rho(H), rho(S)), and enthalpic and entropic substituent constants (sigma(H), sigma(S)). A satisfactory linear free energy relationship (LFER) with the Hammett type equation has been obtained. The reaction constants have been calculated for the different EtOH-H2O mixtures and several reasonable quantitative equations have been obtained to relate them with the composition or dielectric constant of the mixed solvents. (C) 2004 Elsevier B.V. All rights reserved.