The ionization enthalpies, Delta H-o, of benzoic acid, o- and p-fluorobenzoic acids were measured calorimetrically at 298.15 K in water-dimethylformamide (DMF) mixtures ranging from pure water up to 0.7 weight-fraction DMF. The ionization Gibbs energies, Delta G(o), of the acids in the same solutions were measured by the conductance method. The corresponding entropies, Delta S-o, of ionization were calculated. The effect of solvent and substituent on the thermodynamic properties of ionization were considered.