Abstract An extended UNIQUAC-Debye-Hückel model is used to predict solid-liquid equilibria for aqueous solutions of nitrate salts with the cations K +, Mg 2+ and Ca 2+. In addition to a model for activity coefficients in mixed electrolyte solutions, the method requires knowledge of the solubility products of all possible solid phases. Furthermore, some ternary solid-liquid equilibrium data are required. It is shown that the extended UNIQUAC equation can be applied to the calculation of solid-liquid equilibria in the K +-Mg 2+-Ca 2+-NO 3 −-H 2O system with good results. A new algorithm for calculation of solid-liquid equilibrium in ionic solutions has been proposed. The number and amount of phases in equilibrium are found by minimization of the total Gibbs function of the system.