The ionization enthalpies of Delta H degrees of o-fluoro and o-chloro benzoic acids were measured by LKB 2277 BioActivity Monitor at 298.15 K in water-ethanol mixtures from pure water up to 0.8 weight fraction ethanol. The ionization-free enthalpies Delta G degrees of the two acids in the same mixtures were obtained from published literature. The corresponding entropies Delta S degrees of ionization were calculated from the values of Delta H degrees and Delta G degrees.