Solubilities of the coal derived solids naphthalene and biphenyl in the solvents benzene and cyclochexane were measured from the temperature near to the melting point of the solutes to that of the solvents, and then the solid-liquid phase diagrams were constructed from the experimental data. The solubilities were accurately predicted by estimating the activity coefficients in the solid solubility equations using the Scatchard-Hildebrand regular solution theory. For the benzene solutions which were close to an ideal solution, the accuracy could be achieved by using the physical properties of the pure components only. For the cyclohexane solutions which were highly non-ideal, however, binary parameter l_ij was needed to improve the accuracy of prediction. The eutectic points of the binary systems were also closely estimated by the method.