A new semi-empirical hydration model (SEHM) for describing electrolyte solutions is proposed. The SEHM model combines the Pitzer-Debye-Huckel theory with Schonert's stepwise hydration model and is able to fit accurately the activity coefficients of strong electrolyte solutions using only one regression parameter. Furthermore SEHM is able to provide accurate estimations of the solutions' water mole fractions, which are verified by the model's ability to theoretically predict the solutions' osmotic coefficients, without resorting to the Gibbs-Duhem equation. (c) 2006 Elsevier B.V. All rights reserved.