In this study, the non-primitive mean spherical approximation (NPMSA) model has been modified and an equation is obtained for calculating the cationic diameter for single electrolyte solutions at various concentrations.Furthermore, based on this modification of the NPMSA model an equation for the Helmholtz energy is derived and the thermodynamic properties are calculated at various ionic strengths and dipole diameters. The results show that the pair potential and electrostatic forces play the main role in the obtained thermodynamic properties.Also, the calculated thermodynamic properties by the NPMSA model are compared with results from other models such as primitive mean spherical approximation (PMSA) and restricted mean spherical approximation (RMSA). The results indicate that the NPMSA model underestimates the thermodynamic properties due to the solvent effects in this model. (C) 2003 Elsevier Science B.V. All rights reserved.