In past years, the complexity of phase equilibrium of mixtures established during many years have prevented the in-depth understanding of their thermodynamics required for successful design and control of the process. Phase behavior describes the complex interaction between physically distinct, separable portions of matter called phases that are in contact with each other. Phase-equilibrium thermodynamics seeks to determine properties such as temperature, pressure, and phase compositions that establish themselves once all tendencies for further change have disappeared. The present article demonstrate the fundamentals of phase-equilibrium thermodynamics of aromatic compound and bulkier group substituted aromatic compound (toluene and para-tert-butyltoluene) used in pharmaceutical industry, especially where the vapor-liquid phase behavior is required. Therefore, the phase behavior of binary systems is more important in the practical situation because the components coexist. The theories of present investigated system is beneficial for academic and industrial point of view for the similar kind of systems to be investigated.