The phase diagrams of liquid–liquid (L–L E), vapor–liquid (V–LE) and even liquid–solid (L–S E) equilibria have been widely stud-ied[1–4]. However, if the equilibrium in question is between asolid, a vapor and two liquid phases the situation is not as wellunderstood. In fact, there is scant literature on this type of system.For instance, it is not even mentioned among all the phase equilib-ria types in the well-known book by John E. Ricci[5].The study of the vapor–liquid–liquid–solid equilibrium (VLLSE)in systems with electrolytes is complex due to the number ofphases involved. Consequently, there are no complete studies thatexamine the shape of the different equilibrium surfaces andregions. In addition there is a lack of reliable VLLSE data, in partdue to the difficulty of finding equipments capable of separatingand analyzing the different phases in equilibrium. On the otherhand, due to the importance of the application of such systems, dif-ferent groups are working on the development of more flexible andreliable thermodynamic models that can be used to improve indus-trial processes. However, experimental data are needed to checkthe predictions and correlations of the models.