Propionic acid is one of the materials that are used for making cellulosic solvent in pharmaceutical industries. Little information of equilibrium data for this material is available in the literature. In this research the liquid-liquid equilibrium data for the ternary system of (water + propionic acid + diethyl ketone) was measured at 25, 35, 45 degrees C. Complete phase diagrams were obtained by determining solubility and tie-line data. The reliability of the tie-line data was ascertained by using the Othmer and Tobias correlation. Distribution coefficients and separation factors were evaluated for an immiscibility region. The experimental tie-line data were also correlated with NRTL and UNIQUAC models and binary interaction parameters of these models have been reported. Both models satisfactorily fit obtained experimental data.