This paper discusses the binary system between para-bromochlorobenzene and para-dibromobenzene. The phase diagram and crystalline parameters of the molecular alloys of this system have been determined by DSC and X-ray diffraction. The miscibility versus temperature study has shown a miscibility in all proportions in the solid state between room temperature and the melting points. The evolution of the alloy crystalline parameters with the composition shows that the deformation of the para-bromochlorobenzene (pBCB) by introducing para-dibromobenzene (pDBB) takes place mainly in the a direction. © 1989.