The phase diagram of urea-4-bromo-2-nitroaniline system was studied which shows a large miscibility gap and the formation of a eutectic and a monotectic where the mole fractions of 4-bromo-2-nitroaniline are 0.899 and 0.020, respectively. The critical temperature is being 61.5 degrees C above the monotectic horizontal. The formation of solid solution of 4-bromo-2-nitroaniline in urea was studied by powder X-ray diffraction technique. Using differential scanning calorimetry (DSC), the enthalpies of fusion of the pure components, the eutectic and the monotectic were determined. The different thermodynamic parameters such as heat of mixing, entropy of fusion, roughness parameter. interfacial energy and excess thermodynamic functions were calculated utilizing the heat of fusion values. The solid-liquid interfacial energy has also been calculated and their effects on morphological change of microstructure of monotectic and eutectic have been discussed. (C) 2011 Elsevier B.V. All rights reserved.