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The solid-liquid equilibrium of the quaternary system Na+//,K+//HPO42-,Cl--H2O and its subsystems Na+// HPQ(4)(2-),Cl--H2O, Na+, K+//HPO42-, Cl--H2O, Na+,K+//HPO42--H2O at 298.2K were investigated by the isothermal solution saturation method. The solubility of salts and the equihbrium solid phases were determined. The ternary phase diagram, quaternary dry-salt phase diagram, and the water-content diagram were constructed. There is one invariant point, two univariant curves, and four crystallization regions in the ternary system Na+//HPO42-,Cl--H2O arid K+//HPO42-,Cl--H2O. The ternary system Na+,K+//HPO42--H2O contains two invariant points, three univariant curves, and six crystallization regions. The quaternary system Na+,K+//HPO42-,Cl--H2O contains three invariant points, seven univariant curves, and five crystallization regions corresponding to KCl, NaCl, Na2HPO4 center dot 12H(2)O, Na2HPO4 center dot 7H(2)O, and K2HPO4 center dot 3H(2)O. In the quaternary system, KCl occupies the largest crystallization region while K2HPO4 center dot 3H(2)O crystallization region is the smallest. Three kinds of hydrated salts were found in the system while there is no double salts or solid solution. HPO42- is the main factor that affects the relative water content in the liquid phase. All the data and results obtained in this paper are of great significance to the crystallization technology of K2HPO4 and the new production process of KH2PO4.