In this work, the water activities of the NaCl-CaCl2-H2O and KCl-CaCl2-H2O ternary systems and of their sub-binary systems NaCl-H2O and KCl-H2O were measured using an isopiestic method at 323.15 K The isopiestic composition line for the NaClCaCl2-H2O system was found to obey the Zdanovskii rule very well, whereas the KCl-CaCl2-H2O system deviated slightly. The addition of NaCl to an aqueous solution of CaCl2 decreased its water activity for all CaCl2 molalities. However, KCl was found to decrease and increase the water activity at low and high CaCl2 molality, respectively. The turning point appears at 8.0 mol.kg(-1) CaCl2 solution. The Pitzer-Simonson-Clegg (PSC) model was applied to represent the water activity of the two ternary systems, and the calculated results are discussed.