The ternary system (NaH2PO4 + Na2SO4 + H2O) at 298.15 K was investigated by using an isothermal solution saturation method. The solubilities and densities of the solution were determined. According to the experimental results, the phase diagram and the diagram of densities versus composition were plotted. The phases of the equilibrium solids were analyzed by X-ray diffraction (XRD), and crystalline regions of both solid phases were determined. It was found that there were two invariant points (NaH2PO4 center dot 2H(2)O + Na2SO4; Na2SO4 + Na2SO4 center dot 10H(2)O), three univariant curves, and three crystallization regions corresponding to sodium dihydrogen phosphate dihydrate (NaH2PO4 center dot 2H(2)O), sodium sulfate (Na2SO4), and sodium dihydrogen phosphate decahydrate (Na2SO4 center dot 10H(2)O; also known as Glauber's salt) in this ternary system. The system belonged to a simple eutectic type, and neither double salts nor solid solutions were formed. This research provided missing information of the phase diagram in this ternary system, which in turn can provide fundamental data for industry and future study. These fundamental data could be useful for separation and crystallization processes in chemical industry.