The methodology for the study of the liquid-liquid-solid equilibrium of quaternary systems including two inorganic salts, water and an organic solvent has been reviewed and applied to the water + sodium chloride + potassium chloride + 1-butanol quaternary system at 25 degreesC. The different equilibrium regions have been systematically studied. The experimental equilibrium data have been used to check the accuracy of the predictions using the electrolyte NRTL model (MNRTL) and the parameters obtained by correlation of ternary systems. The results obtained with the model are satisfactory. (C) 2003 Elsevier B.V. All rights reserved.