Proper evaluation of physical properties of aqueous solutions is essential in the analysis of desalination systems. While standard seawater property data are readily accessible, they are generally not accurate for aqueous solutions requiring desalination that have significantly different composition than seawater. Since experimental data for a given solution may be unavailable under the conditions of interest, thermodynamic models are needed for relevant physical properties, particularly, activity and fugacity coefficients. Effects of composition and nonidealities in mixed electrolyte solutions are considered through a parametric study of the least work of separation. Conditions under which existing single electrolyte solution models, including ideal solution approximation, Debye-Huckel theory (Davies equation), and Pitzer's ionic interaction model, are valid when analyzing mixed electrolyte solutions are examined by comparing them to the Pitzer-Kim mixed electrolyte model. It is found that single electrolyte models often result in greater error than the ideal solution approximation when studying all but the most dilute mixed electrolyte solutions. Additionally, an effective molality can be used with the Pitzer model to increase the accuracy of the single electrolyte model as applied to mixed electrolytes. Finally, composition is a significant variable in the overall work of separation requirements. (C) 2013 Elsevier B.V. All rights reserved.