Liquid-liquid extraction using ethers as solvents is a potentially energy saving alternative for the concentration of aqueous ferric chloride solutions. Adequate thermodynamic models that describe the behavior of the resulting quaternary systems (FeCl3, ether, acid and water) are not available in the literature. In this paper, the development of an equilibrium description applying the NRTL-model is presented, including experimental validation and fitting of the NRTL-parameters on the validated data. Equilibrium experiments were performed for the ternary systems (water + HCl + DiPE) and (water + FeCl3 + DiPE) and the obtained data is in good agreement with the results from Maljkovic et al. [38] and Cambell et al. [39]. Experimental data of the quaternary system is taken from Maljkovic et al. [37]. The obtained binary interaction parameters to describe the (liquid + liquid) quaternary system (water + FeCl3 + HCl + DiPE) and the constituting ternaries by the NRTL model are presented. Model predictions are in good agreement with the experimental data. (c) 2013 Elsevier Ltd. All rights reserved.