This paper is devoted to the consideration of isotherm-isobaric solubility diagrams of quasi-simple systems. Quasi-simple systems are multicomponent systems in which the lines (surfaces, hypersurfaces) of constancy solvent activity are characterized by a linear interdependence of the variables of the composition of liquid solutions. The specific type of dependence of the incomplete Gibbs potential on the composition variables for such systems is determined. A universal algorithm for the calculation of isotherm-isobaric solubility diagrams of these systems, independent of the valence type of the electrolyte, the number of components of the system, and the types of solid phases, which is also extended to systems with solid solutions formed, is presented. The results of nonmodel thermodynamic calculations of solubility diagrams are in convincing agreement with experimental data available in the literature.