The osmotic coefficients of aqueous calcium chloride solutions were experimentally determined at T = 313.15 K by the isopiestic method. Magnesium chloride served as the isopiestic standard for the calculation of osmotic coefficients. The molality range covered in this study correspond to about 0.1 mol . kg(-1) to 3.0 mol . kg(-1). In addition, the osmotic coefficients of aqueous mixtures of calcium chloride and magnesium chloride were determined over the range of ionic strength levels of about 0.1 mol . kg(-1) to 9 mol . kg(-1) and at various mole fractions. The results obtained were correlated by the Pitzer equation.