The calculation of thermodynamic parameters for the modeling of natural brines contain-ing Br– was carried out. Excess functions were represented by the Pitzer’s model (for the liquid solutions) and the regular solutions model (for the solid solutions). The five-component recipro-cal water-salt system Na+, K+, Mg2+║ Cl–, Br– – H2O at 25oC was considered as the basic one. For this system and for all its subsystems – 5 quaternary systems (Na+, K+║ Cl–, Br– – H2O, Na+, Mg2+║ Cl–, Br– – H2O, K+, Mg2+║ Cl–, Br– – H2O, Na+, K+, Mg2+║ Cl– – H2O, Na+, K+, Mg2+║ Br– – H2O) and 9 ternary systems – equilibrium phase diagrams (isotherms of solubility) were calculated. In some of these diagrams fields or curves of crystallization of different solid solu-tions (formed by individual salts, its hydrates or ternary compounds) exist. The results of phase equilibrium calculation are fairly well consistent with experimental solubility data.