The isopiestic method has been used to determine the osmotic coefficients of the binary solutions MgBr2(aq) (from 0.4950 to 2.5197 mol kg(-1)) at the temperature T = 323.15K. Sodium chloride solutions have been used as isopiestic reference standards. The solubility of the bromide minerals in the mixed system (m(1)KBr + m(2)MgBr(2))(aq) has been investigated at T = 323.15 K by the physico-chemical analysis method. In addition to simple salts {KBr(cr) and MgBr2 center dot 6H(2)O(cr)}, equilibrium crystallization of the highly incongruent double salt with stoichiometric composition 1:1:6 {bromcarnallite: KBr center dot MgBr2 center dot 6H(2)O(cr)} was also established. The results obtained from the isopiestic and solubility measurements have been combined with all other experimental thermodynamic quantities available in the literature (osmotic coefficients, and solubility of the bromide mineral) to construct a chemical model that calculates solute and solvent activities and (solid + liquid) equilibria in the MgBr2(aq) binary, and (m(1)KBr + m2MgBr2)(aq) mixed systems from dilute to high solution concentration within the (273.15 to 438.15) K temperature range. The solubility modelling approach based on fundamental Pitzer specific interaction equations is employed. It was found, that the standard for 2-1 type of electrolytes approach with three (beta((0)), beta((1)), and C-phi) single electrolyte ion interaction parameters gives excellent agreement with osmotic coefficients from T = (298.15 to 373.45) K; up to saturation at 298.15 K, and up to m(MgBr2) = 5.83 mol . kg(-1) at 373.45 K, and with MgBr2 center dot 6H(2)O(cr) equilibrium pure water solubility data within the (273.15 to 438.15) K temperature range and up to approximate to 8.5 mol . kg(-1) used in parameterization. The model for the ternary system gives very good agreement with the equilibrium solubility for bromide salts, which are available from T = (273.15 to 323.15) K. According to the model predictions, bromcarnallite is a stable phase in the (m(1)KBr + m(2)MgBr(2))(aq) system from T = (273.15 to 342.15) K. The model predicts that at higher temperature the mixed system is of a simple eutonic type. Limitations of the mixed solutions model due to data insufficiencies at high temperature are discussed. (C) 2010 Elsevier Ltd. All rights reserved.