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Thermal analysis technique is a powerful tool for the study of various disciplines in thermodynamics and dynamics[1].Heat-flux differential scanning calorimetry (hf-DSC) commenced in the mid-sixties by introducing a thermally inert substance (reference material) and measuring the heat flux difference between the sample and the reference material in the heating-curve method,which gradually gained acceptance in the fields of minerals,metals,ceramics,electronics materials,polymers,organic substances,inorganic substances,pharmaceuticals,food stuff and biological organisms [2-3].In phase chemistry,the phase diagram should be drawn at first when researching the phase morphology of binary or multicomponent system.Nowadays,the quantitative analysis of DSC heat flow rate measurement is one of the useful methods for phase diagram investigations in some complex systems [4-5] In this paper,the phase equilibria of the ternary system (Na.2PHO4·12H2ONa2SO4 ·10H2O) were investigated by DSC.The phase diagram drawn according to the transformation temperature and enthalpy of different components is shown in Figure 1.Basing on the quantitative analysis of DSC heat flow measurements,the eutectic point and the compositions of eutectic can be sensitively determined.The experiment results show that Na2PHO4·12H2O-Na2SO4·10H2O behaves as a eutectic system,its eutectic point occurs 62% Na2PHO4·12H2O and 38% Na2SO4·10H2O at 298.75K.