Solid-liquid equilibria (SLE) data have been measured for the quaternary systems of aqueous solution + binary fatty acid. In addition, binodal data of two liquid phases have also been obtained. Aqueous acetone solution and aqueous ethanol solution were examined as a solvent for the binary lauric acid + myristic acid system. The co-solvent effect of acetone and ethanol could be investigated on the SLE for the binary fatty acid system. However, the anti-solvent effect of water is slightly observed on the SLE for the ternary systems. The binodal region of two liquid phases became larger when the ratio of myristic acid to the total fatty acids increased. The aqueous acetone solution made the binodal region slightly larger than the aqueous ethanol solution. The NRTL equations were examined for the prediction and correlation of the measured SLE and binodal data. The NRTL parameters from the binary systems could describe the SLE phase diagram qualitatively. However, they could not present the binodal curve of two liquid phases. The correlations of SLE and binodal data could be satisfied by the NRTL equation.