The phase equilibrium of the ternary system (KH2PO4+ NaH2PO4 + H2O) at 303.15 K was investigated by a combination of the isothermal solution saturation method and the moist residues method. The solid phase formed in the equilibrium state was verified by X-ray diffraction (XRD). A phase diagram and a densities versus composition diagram were constructed on the basis of the experimental data of the solubility and densities. It was discovered that there was one invariant point (KH2PO4 + NaH2PO4 center dot 2H(2)O), two single saturated liquid curves, and two fields of crystallization corresponding to KH2PO4 and NaH2PO4 center dot 2H(2)O, with no double salt forming in this ternary system. Our research obtained new experimental data for this ternary system and gives guidance for potassium dihydrogen phosphate preparation and separation.