The ternary parameters of the Pitzer equations and solubility diagrams of three-component water-salt systems of the type H+, Men+ ∥ Cl--H2O (Men+ = Na+, K +, Mg2+, Ca2+, Sr2+, and Ba 2+) were calculated in the temperature range 0-40°C, corresponding to the conditions of formation of natural brines. The calculation results were in convincing agreement with the available experimental solubility data.