Liquid−liquid equilibria (LLE) data of the quaternary system {methyl butyl ketone + phenol + hydroquinone + water} and two relevant ternary systems, {methyl butyl ketone + phenol + water} and {methyl butyl ketone + hydroquinone + water}, were measured at T = 313.2 K and p = 101.3 kPa. The experimental data were correlated with the nonrandom two-liquid (NRTL) and universal quasichemical (UNIQUAC) models. The root-mean-square deviation of the NRTL and UNIQUAC models were 0.144 and 0.0476%, respectively. Good agreement was found between experimental LLE data and those calculated from the NRTL or UNIQUAC equations for all systems studied in this work.