This work introduces a new tool able to predict water activities and activity coefficients of electrolytes in binary {water-electrolyte} systems. In mixtures containing electrolytes, the system is characterized by the presence of both molecular and ionic species, resulting in three different types of interactions: ion-ion, molecule-molecule and ion-molecule. Ion-ion interactions are governed by electrostatic forces between ions that have a much longer range than other intermolecular forces. The long range interactions between ions are taken in account by the Pitzer term based on the Debye-Huckel theory. Molecule-molecule and ion-molecule interaction forces are known to be short-range in nature. To determine short range mean activity coefficients of salts in {water-electrolyte} binary mixtures, a chemical treatment of ions solvation is combined with the predictive power of the COSMO-RS model. The main originality of this work resides in this chemical treatment model that provides the thermodynamic relations which enable to determine the equilibrium properties of the real solution {water-salt}, knowing those of a hypothetical mixture containing water and hydrated clusters. The resulting model called "COSMO-RS-PDHS" predicts results that are in good agreement with experimental data. (C) 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.