Liquid-liquid equilibrium data of water + formic acid + acetic acid + solvent (amyl acetate, diisobutyl ketone, and diisopropyl ether) quaternary systems were measured at 298.15 K and atmospheric pressure. Complete phase diagrams were obtained by determining solubility curves and tie-lines. A comparison of the extracting capabilities of the solvents was made with respect to distribution coefficients, separation factors, and solvent-free selectivity bases. Reliability of the data was ascertained from Othmer-Tobias plots. The tie-lines were also predicted using the UNIFAC model. It is concluded that the used solvents can be treated as suitable separating agents for dilute aqueous formic and acetic acid mixtures.