AbstractThe influence of simplifying assumptions of the electrolyte‐nonrandom two‐liquid (NRTL) model in the derivation of activity coefficient expressions as applied to multi‐electrolyte systems is critically examined. A rigorous and thermodynamically consistent formulation for the activity coefficients is developed, in which the simplifying assumption of holding ionic‐charge fraction quantities constant in the derivation of activity coefficient expressions is removed. The refined activity coefficient formulation possesses stronger theoretical properties and practical superiority that is demonstrated through a case study representing the thermodynamic properties and speciation of dilute to concentrated aqueous sulfuric acid solutions at ambient conditions. In this case study phenomena, such as hydration, ion pairing, and partial dissociation are all taken into account. The overall result of this study is a consistent, analytically derived, short‐range interaction contribution formulation for the electrolyte‐NRTL activity coefficients and a very accurate representation of aqueous sulfuric acid solutions at ambient conditions at concentrations up to 50 molal. © 2008 American Institute of Chemical Engineers AIChE J, 2008