A remarkable disadvantage of pure ionic liquids, to be used as a solvent in different processes, is their high viscosity and density. Here, n-butyl acetate was used as a cosolvent in the liquid-liquid system of {water+acetone+1-hexyl-3-methylimidazolium hexafluorophosphate (HMIMPF6) ionic liquid}. Mixtures of the ionic liquid and the cosolvent were used as the organic phase to extract acetone from aqueous phase. A drastic decrease in viscosity and a significant decrease in density of the ionic liquid were achieved in the presence of cosolvent. Accordingly, the liquid-liquid equilibrium of different provided systems was studied under temperature of 298.2K and atmospheric pressure of 81.5kPa. Binary phase regions and corresponding tie-lines and binodal curves were obtained for ternary and quaternary systems. Results show the extension of the binodal solubility region as well as enhancements up to 41.8% in the acetone separation factor. The tie-line data consistency was confirmed by the Othmer-Tobias and the Hand equations. Moreover, the nonrandom two-liquid model of activity coefficient was applied satisfactorily to reproduce the equilibrium data with a root-mean-square deviation of only 1.95%.