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During the biodiesel production process, generally, two purification steps are performed. The first is a settling process in order to remove the main byproduct (glycerol). Nevertheless, this simple stage is insufficient to achieve purity levels required for commercial specifications. Second, a water washing is used to remove an excess of alcohol and other contaminants. Given the importance of equilibrium data for operation and optimization for liquid−liquid extraction unit operation, the study of liquid−liquid equilibrium (LLE) for methylic cottonseed biodiesel + methanol + water and ethylic cottonseed biodiesel + ethanol + water systems are studied at (293.15 and 313.15) K at atmospheric pressure. LLE data have been quantified by intermediate of two different techniques (titration and densimetric). From these experimental data, distribution coefficients and selectivity have been calculated. As expected, a good extraction capacity of water to remove alcohol has been observed. The reliability of the experimental data has been verified by the correlation proposed by Othmer−Tobias. In order to describe the LLE for the experimental systems studied, three predictive thermodynamic models were used, as follows: UNIFAC, UNIFAC-LLE, and UNIFAC-Dortmund. A root-mean-square deviation (RMSD) has been used as the statistical parameter, and it shows the follow deviations: 3.27 %, 3.62 %, and 4.00 % for the models UNIFAC, UNIFAC-Dortmund, and UNIFAC-LLE, respectively. Therefore, best representation of the LLE systems followed the order: UNIFAC > UNIFAC-Dortmund > UNIFAC-LLE.

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