Abstract: In a binary system involving an intermediate compound,if the liquidus has been measured, if the entropy or enthalpy of furmation of intermediate compound has been known, and if the entropy of the liquid phase is known or may be assumed to be ideal, then an exact thermodynamic calculation of the properties of the liquid phase can be made Reversely, if the thermodynamic properties of the liquid phase have known, then the liquidus can be exactly calculated by using only the entropy or enthalpy of formation of intermediate compound. In this paper, general relationships between the phase diagram and thermodynamic properties have been developed, five quantities is sufficient to permit the other two to be calculated exactly: the liquidus, enthalpy of the liquid, entropy of the liquid, enthalpy of formation of intermediate compound, entropy of formation of intermediate compound.