Liquid-liquid equilibria data of the quaternary system methyl isobutyl ketone (MIBK)-water-phenol-hydroquinone were measured at 25 degrees C under atmosphere pressure. The experimental data were correlated with the UNIQUAC and NRTL activity coefficient models on the base of the fixed binary interaction parameters that were obtained from two sub-ternary systems MIBK-water-phenol and MIBK-water-hydroquinone. The root mean square deviations (RMSD) show that the regressed results for the quaternary system were in good agreement with the experimental data for both UNIQUAC and NRTL models. The comparison between experimental and calculated distribution coefficient values of phenol and hydroquinone shows that a relative deviation of less than 5% is obtained. (c) 2007 Elsevier B.V. All rights reserved.