Liquid-liquid equilibria of mixed solvent electrolyte systems were correlated using an extended electrolyte NRTL model. The electrolyte NRTL expression for the activity coefficients has been modified by taking into account the derivatives to the solvent composition of the physical properties and by a new Bronsted-Guggenheim (BG) expression. In addition to some ternary mixed solvent electrolyte systems, the model has been applied to the quaternary systems that are of importance in the extraction process of epsilon-caprolactam: water + epsilon-caprolactam + solvent + ammonium sulfate (AS), The Liquid-liquid equilibria of the ternary and quaternary systems involved and the mean activity coefficients of the salt + water systems were used simultaneously to obtain the adjustable parameters. The results are compared to the original electrolyte NRTL of Chen.