The synergistic effect for a solvent mixture of 2-pentanone and n-pentanol to extract hydroquinone from aqueous solution was investigated. Liquid-liquid equilibria data for the quaternary (2-pentanone + n-pentanol + hydroquinone + water) system were measured under atmospheric pressure at 298.2 K, using various compositions of solvent mixture. The tie-line length was calculated to estimate the size of the immiscibility zone. The distribution coefficient and separation factor were calculated to evaluate the extraction efficiencies. The experimental data were correlated with NRTL and UNIQUAC models, and the corresponding binary interaction parameters were obtained.