This paper, written in celebration of Professor Robin H. Stokes' 90th birthday anniversary, describes isopiestic measurements and the development of a thermodynamic model for the highly important MgCl2 + CaCl2 + H2O system at (25 and 50) degrees C. The isopiestic method has been used to determine the osmotic coefficients of the mixed solutions from low to high concentrations. Sodium chloride and sulfuric acid solutions have been used as isopiestic reference standards. The isopiestic results obtained have been combined with all other experimental thermodynamic quantities available in the literature (osmotic coefficients, water activities, calcium and magnesium chloride mineral's solubilties) to construct a chemical model that calculates solute and solvent activities and solid-liquid equilibria in the MgCl2 + H2O and MgCl2 + CaCl2 + H2O systems. The mixed solution model created here used CaCl2 + H2O system parametrization established in our previous study (Christov and Moller, GCA 2004b, 68, 3717). The model incorporates the concentration-dependent specific interaction equations of Pitzer for aqueous solutions (J. Phys. Chem. 1973, 77, 268). The model for the binary MgCl2 + H2O system is validated by comparing activity coefficient predictions with those given in the literature and not used in the parametrization process. The model predictions on the effect of mixing on the deliquescence relative humidity in MgCl2 + CaCl2 + H2O solutions saturated with highly soluble MgCl2 center dot 6H(2)O(cr), CaCl2 center dot 6H(2)O(cr), and 2MgCl(2)center dot CaCl2 center dot 2H(2)O (cr) sea-salt minerals are also given.