A new theory of electrolyte solutions is discussed which, as opposed to the Debye-Huckel method valid for relatively small concentrations only, enables one to describe the thermodynamic properties of electrolyte solutions over a wide range of state parameters. The novelty of the proposed theory is that it takes into account the dependence or hydration numbers upon concentration, the fact that failed to attract much attention before. The applicability of the theory to the calculation of activity coefficients of some electrolytes (LiCl, NaCl, KCl, RbCl, and CsCl) is demonstrated. A method is proposed for the determination of the thermodynamic parameters of solvation of separate ions. (C) 2008 Elsevier Ltd. All rights reserved.