The procedures developed in this paper have been employed to calculate theoretical free energies of formation of ion-water clusters for comparison with experiment. Gibbs free energies were calculated for the gas phase reaction, ion(H2O)N-1+H2O(vapor)=ion(H 2O)N, for the Li+, Na+, K +, Cl-, and F- ions and for N=1-6. The standard state for all calculations was taken as 298°K and 1 atm. The Monte Carlo method was used to evaluate the appropriate classical expressions of statistical mechanics by employing the intermolecular potential functions recently developed from ab initio Hartree-Fock calculations. Enthalpies, entropies, and structural information were also calculated. Agreement with experiment is sufficiently good to demonstrate the feasibility of this approach. Copyright © 1976 American Institute of Physics.