In this work, we propose an extension to electrolyte systems of the Cubic Plus Association (CPA) equation of state. The effect of the ionic charge is accounted for by a Born term, and a Mean Spherical Approximation (MSA) contribution for the coulombic interactions between permanent charges. The effect of the Born term on the excess properties is directly related to the change of the dielectric constant with concentration. This paper analyses the capacity of the model to represent the properties of aqueous solutions by comparison with experimental data of apparent molar volume and activity coefficients. In a first stage, we discuss the contribution of each term based on the water-NaCl and water-CaCl2 systems. Simonin's model for the dielectric constant is found most convenient for describing the dielectric constant. In a second stage, the model is extended with success to a set of ten different salts in water. (C) 2008 Elsevier B.V. All rights reserved.