Solubilities and physicochemical properties (density and refractive index) for the ternary system (Na+//SO42-, BO2--H2O) at (288.15 and 308.15) K and 101.325 kPa were studied by the isothermal dissolution equilibrium method. Phase diagrams and diagrams of the physicochemical properties (density and refractive index) were plotted against the concentration of sodium sulfate. For both temperatures, the phase diagrams contained one invariant point, two univariant curves, and two crystallization regions corresponding to sodium metaborate tetrahydrate (NaBO2 center dot 4H(2)O, Nb) and mirabilite (Na2SO4 center dot 10H(2)O, Mir) at 288.15 K, and Nb and thenardite (Na2SO4, Th) at 308.15 K, respectively. The area of the crystallization region of sodium metaborate tetrahydrate was enlarged with the increase of temperature, and the solid phase of mirabilite transformed into thenardite when the temperature increased from 288.15 to 308.15 K. The physiochemical properties including density and refractive index of the system at (288.15 and 308.15) K were similarly increased first and decreased later with the increase of sodium sulfate concentration in the equilibrium solution. The experimental data of the density and refractive index were correlated using the empirical equations, and the calculated and experimental values agreed well.