Abstract: Specific thermodynamic models for a quaternary system of acetic acid-ethanol-water-ethyl acetate are established. The system is a strong polar mixture. With the exception of acetic acid azeotropic compounds exist in between the three other components. Furthermore, association of acetic acid in the vapor phase occurs in the system. All these features, tending to make calculations more camplex than ever, are taken into consideration. The ASOG combined model is chosen for calculating vapor-liquid equilibria, as well as for liquid-liquid equilibria and enthalpies.In calculating vapor-liquid and liquid-liquid equilibria, results from ASOG model are com pared with those from UNIFAC model. The thermodynamic models presented in this paper correlate very well with about 150 experimental data and are successfully used in the simulation of an industrial tower integrated with a reboiler as a reactor.