A method based on interval analysis is presented for determining parameter values from mutual solubility data in two-parameter activity coefficient models for liquid-liquid equilibrium. The method is mathematically and computationally guaranteed to locate all sets of parameter values corresponding to stable phase equilibria. The technique is demonstrated with examples using the NRTL and electrolyte-NRTL (eNRTL) models. In two of the NRTL examples, results are found that contradict previous work. In the eNRTL examples, binary systems of an ionic liquid and an alcohol are considered. This appears to be the first time that a method for parameter estimation in the eNRTL model from binary LLE data (mutual solubility) has been presented. (c) 2007 Elsevier B.V. All rights reserved.