Previous theoretical calculations and experimental measurements of the surface energies or enthalpies of the alkali halides are reviewed briefly. A new attempt to determine the surface enthalpy associated with the {100} face of sodium chloride from a calorimetric study of the effect of particle size on the heat of solution is described. The result (305 ergs/cm.2at 25 °C.) appears to be larger than might be predicted on the basis of the classical Born-Mayer theory.