An apparatus is described for the precise measurement of freezing-point depressions for non-aqueous solutions by a heating curve method. The apparatus has been used for measurements on solutions in dimethyl sulphoxide and in sulpholane (tetrahydrothiophene-1,1-dioxide), the relevant physical properties of which are critically discussed. Osmotic coefficients have been determined with an accuracy of 0.2–1 % for solutions of some alkali metal perchlorates and of lithium chloride. The salts studied behave differently in the two solvents. The ionic solvating power of dimethyl sulphoxide is much greater than that of sulpholane, which results in opposite sequences of osmotic coefficients for the alkali metal perchlorates, and striking differences in degrees of ion association in the two solvents, particularly for lithium chloride. The interpretation of these observations is discussed, together with the behaviour of lithium halides in a range of solvents.