The two immiscible liquid phases in equilibrium with a single liquid phase have been observed during the phase diagram study of an organic analog of a metal-nonmetal system, involving p-dibromobenzene-succinonitrile (DBB-SCN). The phase equilibrium shows the formation of a eutectic and a monotectic, with large miscibility gap in the system, contain 0.982 and 0.057 mole fractions of SCN, respectively, and the consolute temperature is 110 degreesC above the monotectic horizontal. The heat of mixing, entropy of fusion, roughness parameter, interfacial energy and excess thermodynamic functions were calculated based on enthalpy of fusion data determined via differential scanning calorimeter (DSC) method. The solid-liquid interfacial energy and its effect on morphological change of monotectic structure have been discussed as well as the interfacial energy balance between the solid and two liquids in the monotectic reaction. Whereas the microstructures of monotectic and eutectic show peculiar characteristic features.