A novel approach is proposed,for solving the problem of the overestimation of the liquid-liquid equilibrium (LLE) range in mixtures typically, associated with the use of equations of state (EOS) of the van der Waals type. This approach modifies the internal EOS structure by combining the van der Waals and the Dieterici forms. It advantageously uses the drawback of the Dieterici form, which strongly underestimates the LLE range. Thus, the new EOS is capable of generating an appropriate balance for the representation Of vapor-liquid equilibria (VLE) and LLE critical data and therefore represents a major improvement in comparison with typical van der Waals-type EOSs. In addition, it gives accurate representation of the Joule-Thomson inversion curve and predicts critical data for nonpolar mixtures without any binary, parameters. For polar systems, the novel EOS predicts critical data for complete homologous series of mixtures using a pair of binary parameters evaluated from a single system. (c) 2005 American Institute of Chemical Engineers.