We report the results of a Monte Carlo simulation of ion clusters using the polarizable model potentials for hydronium ion and a water molecule proposed by Kozack and Jordan [J. Chem. Phys. 96, 3120 (1992); 96, 3131 (1992)]. The grand canonical Monte Carlo simulation aided by the umbrella sampling technique allows us to evaluate very efficiently the equilibrium distribution of ion clusters of various sizes. Thermochemical data of interest, such as the standard free energy, enthalpy, and entropy of protonated water clusters follow immediately from this distribution.