dProjects on tertiary oil recovery by means of microemulsions have been mainly concerned with, first, the ability of a microemulsion to dissolve oil and water simultaneously and, second, the attainment of very low interfacial tensions. Therefore, the design and analysis of chemical flooding processes for enhanced oil recovery must be based on calculations of phase equilibria for these systems, which are composed of water (brine), oil, surfactant and co-surfactant (usually an alcohol). Consequently, the understanding of phase behavior of these systems is of fundamental importance to the development of any surfactant-based chemical flooding process. The purpose of this work was to give a thermodynamic analytical representation of the phase diagram of microemulsion systems similar to those used in enhanced oil recovery. The algorithms presented for the calculation of multiphase liquid equilibria and the methods for the estimation of the excess Gibbs energy model interaction parameters were successfully tested for the representation of experimental multiphase liquid equilibrium data of an oil-brine-surfactant-alcohol model system. In addition,to represent effectively the phase diagram of this system, an empirical expression was introduced into the selected excess Gibbs energy model to account for the specific role of the surfactant in these complex systems.