AbstractBinary and ternary solubility data were measured for the NaCl‐KCl‐H2O system in the temperature range 293‐360 K using a standard visual method. The solid‐liquid phase diagram of the ternary system was calculated from the binary data and from the activity coefficients of the electrolytes evaluated by the Pitzer model. Some empirical functions of temperature were used to describe the virial coefficients. The calculated ternary solubilities were compared with the experimental values yielding a very good agreement. Therefore, it was possible to conclude that, in the range of concentration and temperature studied, the Pitzer model provides an excellent representation of the activity coefficients, thus allowing the accurate prediction of solubilities in the ternary