This paper describes the development of a thermodynamic model for the highly important MgCl2 + H2O and HCl + MgCl2 + H2O systems at (0 to 100) degrees C. The model incorporates the concentration-dependent specific interaction equations of Pitzer for aqueous solutions (J. Phys. Chem. 1973, 77, 268). All experimental thermodynamic quantities available in the literature (osmotic coefficients, water activities, magnesium chloride mineral's solubilities) are used to construct a chemical model that calculates solute and solvent activities and solid-liquid equilibria in the MgCl2 + H2O and HCl + MgCl2 + H2O systems. The (0 to 100) degrees C model for the binary MgCl2 + H2O system gives excellent agreement with activity and solubility data used in parametrization and/or validation processes. To create a temperature-variable mixed solution model for the HCl + MgCl2 + H2O system, a HCl + H2O model parametrization established in our previous study (Christov and Moller, GCA 2004a, 68, 1309) has been used. To develop an equilibrium Solubility model of hydrogen-carnallite (HCl center dot MgCl2 center dot 7H(2)O(cr)), crystallizing within the HCl + MgCl2 + H2O system at very high acid concentration, we extend the application range of H-Cl pure electrolyte parametrization up to very high molality (approximate to 23 mol.kg(-1) at (0 and 25) degrees C and approximate to 15 mol.kg(-1) at higher temperature). The evaluated T-variable single electrolyte and mixing ion interaction parameters and thermodynamic Solubility products of minerals give an excellent agreement with MgCl2 center dot 6H(2)O(cr) and HCl center dot MgCl2 center dot 7H(2)O(cr) equilibrium solubility data in mixed solutions at (0 to 100) degrees C. The model predictions on the effect of temperature and mixing on the deliquescence relative humidity (DRH/MDRH) in MgCl2 + H2O and HCl + MgCl2 + H2O solutions saturated with highly soluble bishofite and hydrogen-carnallite minerals are also given.