Abstract: To investigate the laws relating to the mole fraction of a component in a binary solution and its vapor pressure, the entropies of dissolution from the solid phase to the liquid phase of 48 typical compounds were assessed and the compounds were confirmed to not decompose during the phase transition processes. Under the condition where a compound also exists in a binary liquid melt, both of the components in the whole concentration range obey Raoult's law. Specifically, there is no such thing as an"activity"as defined by Lewis. The so-called "activity" is actually the balance mole fraction in the case where all components in the system are considered by comparing with the measured activity data for Mg-Si, Sb-In, and Fe-Mn binary systems. It is verified that the calculated equilibrated mole fraction represents the activity, thereby the binary metallic melts involve no activity. In addition, a general formula is deduced for the equilibrium mole fraction in the binary system in the case where the melt contains one intermetallic compound. In particular, the cases corresponding to A₂B-type or AB-type intermetallic compounds were discussed. Finally, a broad sense Raoult's law was proposed that it is applicable in all three regions in binary solution divided by different temperatures:(I) the region corresponding to temperatures above the melting point of the component,(Ⅱ) the region corresponding to temperatures lower than the melting point of the pure component but greater than the liquidus temperature, and (Ⅲ) the region corresponding to temperatures lower than the liquidus temperature.