Monte Carlo bicanonical ensemble computer simulations for clusters consisting of Na+ ion and up to 21 water molecules were carried out with Aqvist ion-water pair additive potential. A number of thermodynamic properties of cluster-water molecule associations were calculated, including the absolute values of the attachment Gibbs free energies. The results demonstrate the validity of 'liquid phase' potential to reproduce adequately well-known Dzidic's and Kebarle [J. Phys. Chem. 74 (1970) 1466] experimental data on the attachment Gibbs free energies and enthalpies for Na+(H2O)n, n = 1,2,...,6 clusters. At the same time, it is necessary to invoke non-additive interactions to achieve an agreement between experimental and computational results for attachment entropy. © 2003 Published by Elsevier Science B.V.