Liquid-liquid equilibrium data of water-acid mixture (formic-propionic acid or acetic-propionic acid)-solvent (amyl acetate, diisobutyl ketone, and diisopropyl ether) quaternary systems were measured at 298.15 K. 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. It is concluded that the used solvents can be treated as suitable separating agents for dilute aqueous formic-propionic acid or acetic-propionic acid mixtures. (C) 2011 Elsevier B.V. All rights reserved.