In order to improve the reliability of purity measurements by differential scanning calorimetry (DSC), the melting process of a test material was studied using a system of ethyl p-aminobenzoate and n-butyl p-hydroxybenzoate. The eutectic mixture began melting accompanying the increase in temperature. After the minor component, p-hydroxybenzoate, had entirely melted in the eutectic mixture, it was confirmed by DSC measurement and solid phase-liquid phase titration that the major component, p-aminobenzoate, which remained as a solid, continued to dissolve into the eutectic mixture. In order to completely grasp the melting image of the test material, it was necessary to measure it from a sufficiently low temperature, and work up to the eutectic peak with the minor component. It was found that, by drawing up a solubility curve, the phenomenon in which p-aminobenzoate dissolves into the melted eutectic mixture is, in this system, close to the ideal solution based on Raoult’s equation, and that the heat of mixing in an infinite dilution was indicated to be a small value of -1.9 J g(-1). For that reason, the heat of fusion and the heat of dissolution coincided within the allowable error. Test materials that have been refined to a certain degree frequently include similar compounds as impurities, and this establishes an approximation of the heat of dissolution with the heat of fusion.