Liquid-liquid equilibrium data of water + acetic acid + propionic acid + solvent ( butyronitrile, benzyl acetate, and methyl isobutyl ketone (MIBK)) quaternary systems were measured. Complete phase diagrams were obtained by determining solubility curves and tie-line data at 298.15 K. The experimental data were compared with predicted values obtained from the UNIFAC method. Selectivity values for solvent separation efficiency were derived from the tie-line data. A comparison of the extracting capabilities of the solvents was made with respect to distribution coefficients, separation factors, and solvent-free selectivity bases. The reliability of the data was ascertained from Othmer-Tobias plots. We conclude that the performance of the solvents increases in the order of butyronitrile, MIBK, and benzyl acetate.