We report liquid-liquid equilibrium data for the quaternary system l-octanol + 2-methoxy-2-methylpropane (MTBE) + water + methanol at 25 degrees C. Experiments were performed with mixtures with bulk compositions equivalent to systems of the form (water + methanol + [m1-octanol + (1 - m)MTBE]), where m = [x(octanol)]/[x(octanol) + x(MTBE)] and takes values of 0 (the ternary system water + methanol + MTBE), 0.2, 0.4, 0.6, 0.8, and 1.0 (the ternary system water + methanol + l-octanol). The quaternary liquid-liquid equilibrium data were correlated by the TK-Wilson, UNIQUAC, and NRTL equations; the UNIQUAC correlation afforded the smallest deviations between experimental and calculated compositions. The quaternary data were satisfactorily predicted by the UNIFAC method.