Liquid-liquid equilibrium (LLE) data were determined for the quaternary system 1-octanol + 2-methoxy-2-methylpropane (MTBE)+ water + ethanol. at 25 degrees C and atmospheric pressure. The full range of compositions was covered by using pseudoternary systems, each one corresponding to a plane in the tetrahedral phase diagram. The tie-lines for the quaternary mixtures did not always lie within the plane for the corresponding pseudoternary system. The LLE data for the quaternary system, and for its constituent ternary subsystems, were correlated using the NRTL (alpha = 0.1, 0.2 or 0.3), UNIQUAC and TK-Wilson equations. Each correlation afforded similar binary interaction parameters for the partially miscible pairs common to the ternary subsystems. Moreover, these parameters differed little from those obtained for these solvent pairs in the quaternary system. The LLE data were predicted using the UNIFAC method and also using the above-mentioned correlation equations, fixing each binary interaction parameter to the value or mean value obtained by correlating the LLE data for the constituent ternary subsystems.