A new set of Pitzer interaction parameters is proposed to describe the excess properties of the H–Na–K–Ca–Mg–NO3–H2O system at 298.15 K. From these parameters we reproduce the osmotic coefficient, mean activity coefficients, and density for the binary systems, and also salt solubility in the ternary subsystems. The binary interaction parameters are either selected from the literature or redetermined in this study. In the case of KNO3–H2O and HNO3–H2O binary systems, partial electrolyte dissociation has to be taken into account to represent accurately all the experimental data up to very high concentrations. Then solubility data of salts in the corresponding ternary systems are used to determine the ternary interaction parameters and the solubility products of the double salts. As KNO3 and HNO3 are partially dissociated, we have determined new interaction parameters involving neutral species (for instance ηKNO3/Na+/H+ or μHNO3/KNO3/Na+) to draw the phase diagram of quaternary systems Na–K–H–NO3. To complement the study, we also propose a set of volumetric Pitzer interaction parameters for all the binary systems and for the H–K–NO3 ternary system, so solution density can be computed at 298.15 K.