The Electrolyte NRTL model proposed by Chen et al. [C.-C. Chen, H.I. Britt, J.F. Boston, L.B. Evans, AIChE Journal 28 (1982) 588-596.] and Chen and Evans [C.-C. Chen, L.B. Evans, AIChE Journal 32 (1986) 444-454.] has been modified to include concentration dependence of interaction parameters in order to enhance the capability of the model in representing the nonideality of concentrated electrolyte solutions. The concentration-dependent interaction parameter is based on the concept that the multi-body effect should be considered in determining the local composition when the electrolyte concentration is high. In this work, a linear concentration dependence is assumed and the activity-coefficient expressions for cations, anions and molecular species are derived from the excess Gibbs energy expression through the thermodynamic relationship. Therefore, the activity-coefficient expressions are consistent and satisfy the Gibbs-Duhem equation. The experimental mean-ionic-activity-coefficients of highly soluble aqueous electrolyte systems have been used to regress the concentration-dependent parameters. The calculated values and experimental data are in excellent agreement. The deviations are usually within experimental uncertainty and significantly smaller than those using the original model. The modified model is completely compatible with the original model because it reduces to the original model when the concentration-dependent term is zero. (C) 1998 Elsevier Science B.V. All rights reserved.