Liquid-liquid equilibria were measured at 25 degrees C for 2,3-butanediol + water + 4-(1-methylpropyl)phenol + toluene. The consistency of the results was checked with the Othmer-Tobias, Bachman, Hand, and Campbell correlations. Results show that distribution coefficients increase with an increase in the concentration of 4-(1-methylpropyl)phenol in toluene, but selectivity remains almost constant. A qualitative interpretation of the quaternary system 2,3-butanediol + water + 4-(1-methylpropyl)phenol + toluene is presented, as at certain concentrations a third phase is formed.