The heats of solution of 18 alkali halides in anhydrous formamide at concentrations of about 0.01 molal and at 25" have been measured with an adiabatic calorimeter. Some physical properties of formamide have been determined. By applying the extended Deboe-Hiickel theory the enthalpies of solution at infinite dilution were calculated. The difference in enthalpies of solution of the fluorides, chlorides, bromides and iodides, respectively, in water and in formamide appeared to be a constant, independent of the cation. An explanation in terms of van Eck's theory of ionic solvation is given.