A generalized electrolyte Non-Random Two-Liquid (eNRTL) model proposed by Chen and coworkers is presented as a comprehensive excess Gibbs energy expression to represent the liquid-phase nonideality for aqueous and mixed-solvent electrolyte systems over the entire concentration range from pure solvents to saturated solutions or fused salts. The eNRTL model is used to correlate mean ionic activity coefficients of mixed-solvent electrolyte systems. The segment interaction concept provides the framework necessary to represent the attractive interaction of ions with the hydrophilic segments of organic solvents and the repulsive interaction of ions with the hydrophobic segments of organic solvents. The ability to explicitly account for such distinctly different ion-segment interactions is shown to be the key for quantitative correlation of mean ionic activity coefficients in mixed-solvent electrolyte systems. (C) 2004 American Institute of Chemical Engineers.