The solubilities in the quaternary system NaCl-KCl-SrCl2-H2O and its subsystem KCl-SrCl2-H2O at 288.15 K were investigated with the isothermal dissolution method. The phase diagram of the ternary system is composed of two single-salt crystallizations for KCl and SrCl2 center dot 6H(2)O, two solubility univariant curves, and one cosaturated point. The dry-salt diagram of the quaternary system consists of three single-salt crystallization fields for NaCl, KCl, and SrCl2 center dot 6H(2)O, three solubility univariant curves, and one invariant point. By combination of the determined solubilities and Pitzer binary parameters obtained in the literature, the lacking Pitzer mixing parameters theta(Na,sr), theta(K,Sr,Cl), psi(Na,Sr,Cl), and psi(K,Sr,Cl) and the solubility product constants of the solid salts for the systems were fitted on the basis of the Pitzer model. The model was then used to calculate the solubilties of the two systems. The theoretical results calculated using the model coincide with the experimental values.