The formation of the hydration shell of chloride ion at room temperature and at a polar stratosphere temperature has been studied by Monte Carlo simulations. Gibbs free energy, entropy and formation work of Cl -(H2O)n, n = 1, ..., 23 clusters are calculated via the bicanonical ensemble method. Pair distribution functions are calculated in n = 70,500 systems. A combined model has been applied: the interaction between the water molecules is described by ST2 potential (F.H. Stillinger, A. Rahman, J. Chem. Phys. 60 (1974) 1545), whereas the description of ion-water interaction is based on SPC model (H.J.C. Berendsen, J.P.M. Postma, W.F. van Gunsteren, J. Hermans, in: B. Pullman (Ed.) Intermolecular Forces, Reidel, Dordrecht, 1981, p. 331). The combined model that has been applied includes explicitly the polarization of the ion in the field of the molecules and the polarization of the molecules in the field of the ion. Ion-water model parameters have been fitted to reproduce the experimental values of free energies and entropies corresponding to the first water molecules association reactions to a cluster in vapour as well as pair distribution functions in bulk phase. The anion hydration shell is thermodynamically stable and remains so in supersaturated vapour. In polar stratosphere conditions, nearly all ions are hydrated by 7-10 water molecules. © 2004 Elsevier B.V. All rights reserved.