Modeling the reactive extraction of carboxylic acids in the presence of inorganic salts requires experimental data for the thermodynamic properties of aqueous solutions of the single salts as well as of aqueous solutions of (salt + carboxylic acid) mixtures. The activity of water in such solutions was determined by isopiestic measurements at 298.15 K for the binary mixture (water + sodium citrate) and the ternary mixtures of (water + citric acid + salt) for the salts sodium chloride, sodium nitrate, sodium sulfate, and sodium citrate. The experimental data are correlated with Pitzer's Gibbs excess energy model for aqueous electrolyte solutions.