Considerable geochemical application may be made of a model or theory which allows the excess Gibbs energy of mixing to be obtained from the properties of the pure components in a system. Such a model predicts phase relations; chemical partitioning between coexisting mineral pairs is completely specified by such a model, provided that the species of interest is one of the components; geothermometric and geobarometric specifications are given, as well as mixing volume, if the model is pressure dependent. The quasi-chemical model of Guggenheim is shown to be appropriate for subsolidus immiscibility in the system NaCl -KC1. This system is analyzed by means of the solvus curve; and the resulting parameters correctly predict the calorimetrically observed mixing enthalpy, entropy, and heat capacity. The predicted partial molar mixing heats agree to within 12 percent with the results of elaborate lattice energy calculations based on a detailed Born-Mayer model with lattice relaxation. The quasi-chemical model may have broad application to other systems of geochemical importance.