Molar free energies of transfer, at 298°K, from water to various methanol–water mixtures of mercuric chloride have been determined through solubility measurements at 298°K and at 318°K. When combined with the known enthalpies of transfer, these free energies yield the corresponding entropies of transfer of mercuric chloride. Values of ΔG_t° decrease steadily with increasing mole fraction of methanol to –1623 cal. mole^–1 at χ(MeOH)= 1, but ΔS_t° attains a maximum value of 1·9 cal. deg.^–1 at χ(MeOH)≃ 0·2 before decreasing gradually to –8·8 cal. deg.^–1 mole^–1 at χ(MeOH)= 1.