The solubilities and physicochemical properties in the quaternary system KCl-NaCl-CaCl2-H2O at 283.15 K have been measured with the isothermal method. The dry-salt phase diagram, water-phase diagram, and physicochemical properties vs composition in the system were plotted according to the measured data. The system contained only the forms of single salts, and no double salts or solid solution were found. On the basis of the extended Harvie-Weare (HW) model and the temperature coefficients of the single-salt parameters obtained from the calorimetric data, the Pitzer parameters for KCl, NaCl, and CaCl2 and the mixed ion-interaction parameters at 283.15 K were obtained. In addition, the average equilibrium constants of the solids at the same temperature were obtained using a method derived from the activity coefficient model of the electrolyte solution theory. Using the average equilibrium constants of the solids at equilibrium, the solubility predictions for the quaternary system are presented. A comparison between the calculated and experimental results suggests that the predicted solubility data obtained with the extended HW model agree well with the experimental data.