The phase equilibrium of the quaternary system Na+, NH4+//Cl-, H2PO4--H2O at 298.15K and 323.15K is investigated by the isothermal solution saturation method. The densities of the saturation solution were obtained. The phase diagrams, water content diagrams and density-composition diagrams were constructed. The phase diagrams of the system contain four ternary invariant points, two quaternary invariant points, five univariant curves and four crystallization regions corresponding to NaCl, NH4Cl, NH4H2PO4 and NaH2PO4 . NH4H2PO4 has the largest crystallization region among the four salts, indicating that NH4H2PO4 is the easiest to crystallize out. There are no double salts or solid solutions in this system. The solubilities of the saturated solutions change regularly with the changing of composition. According to the phase diagrams, the appropriate concentration of Cl- and H2PO4- for the crystallization of pure monoammonium phosphate can be obtained and the crystallization process can be planned. The solubility data, density data, phase diagrams and water content diagrams can provide fundamental basis for the crystallization process of monoammonium phosphate. (C) 2015 Elsevier B.V. All rights reserved.