Published data on the heats of crystallization from aqueous solutions of potassium chloride and magnesium chloride hexahydrate are critically reviewed and compared with those evaluated from solubility and activity data. The recommended values of the heats of crystallization are −13.8±0.1 kJ mole−1 for KCl and −15.8−0.3+2.8 kJ mole−1 for MgCl2·6 H2O. A test of the mutual consistency of data for dissolution heats, temperature dependence of solubility and concentration dependence of activity and/or osmotic coefficient is a side-issue of the review.