Differential heats of solution (L) of water and ethanol in various solvents ranging from pure water to pure ethanol have been determined by measuring heats of solution of small quantities of pure water and ethanol in large quantities of the various solvents. Partial molal entropies, based on the pure liquids as standard states, have been calculated for ethanol in water and water in ethanol. The standard (hypothetical 1 m solution) partial molal entropy, heat of formation, and free energy of formation have been calculated for aqueous ethanol. The entropy data show that addition of small amounts of ethanol to water increases the structuredness of the solutions. Heats of solution of NaCl(c) have been determined in various water-ethanol mixtures. Heats of solution of aqueous HCl and aqueous NaOH have also been determined and used in calculating ΔH values for solution of HCl(g) and NaOH(c) in the various solvent mixtures. The ΔH values for solution of NaCl(c), NaOH(c), and HCl(g) all show maxima in the water-rich region, while those for HCl(g) also show a minimum in the ethanol-rich region. Heats of reaction of NaOH with HCl have been measured and used in calculating heats of ionization of water in the various solvent mixtures. The resulting ΔH values show a small maximum in the water-rich region. Some similar work has been done on water-methanol systems.