Cation-exchange experiments on Si-rich and Si-poor phillipsites with a 0.1 N NaCl + KCl or a 0.1 N NaCl + CaCl2 solution at 70 and 35 degrees C are analyzed to obtain the equilibrium constants and Margules parameters for their solid solutions, The Si-rich phillipsite shows a larger preference for K+ over Na+ than the Si-poor one at 70 and 35 degrees C. While the Si-rich phillipsite shows no preference for Na+ over Ca2+ at 70 degrees C and prefers Na+ over Ca2+ at 35 degrees C, the Si-poor phillipsite prefers Ca2+ over Na+ at both temperatures. Based on the experimental results, equilibrium constants for the cation-exchange reactions and Margules parameters for the phillipsite solid solutions are computed. The equilibrium constants for Na-K and Na-Ca exchange reactions depend strongly on the Si content in the phillipsite, The Si-rich phillipsite shows the large nonidealities in the Na-K series compared with the Si-poor phillipsite, For Na-Ca series phillipsites, the activity coefficient versus composition relations at 70 degrees C do not differ between these phillipsites but those relations at 35 degrees C do show differences.