Tie line data for the ternary system water + methyl tert-butyl ether + ethanol, were obtained at T=303.15 K. A quaternary system containing these three compounds and benzene was also studied at the same temperature. In order to obtain the binodal surface of the quaternary system, four quaternary sectional planes with several ratios of benzene and ethanol were studied. The distribution of methyl tert-butyl ether between the aqueous phase and the organic one was also analysed. For the ternary system, the distribution coefficients decrease from the plait point until reaching the value corresponding to the mutual solubility. A relationship between the mass fraction of methyl tert-butyl ether in the aqueous phase and the concentration of the mixture (ethanol + benzene) in the global composition was also obtained. From the experimental results we conclude that the concentration of methyl tert-butyl ether in the aqueous phase generally decreases when the sum (ethanol + benzene) in the global composition increases for the quaternary sectional planes. Furthermore, the liquid-liquid data for the ternary system were used to obtain interaction parameters for the NRTL and UNIQUAC activity coefficient models. (c) 2006 Elsevier B.V. All rights reserved.