The osmotic and activity coefficients of aqueous magnesium chloride solutions were experimentally determined at T = 313.15 K by the isopiestic method. Lithium chloride served as the isopiestic standard for the calculation of osmotic coefficients. The molality ranges covered in this study correspond to about (0.1 to 2.5) mol.kg(-1). In addition, the osmotic coefficients of aqueous mixtures of magnesium chloride and lithium chloride were determined over the range of ionic strength levels from (0.1 to 7) mol.kg(-1) and at various mole fractions. The results obtained were correlated by the Pitzer equation. From these correlations, the activity coefficients of both salts in the mixture were calculated.