Ionization potentials (IPs) of water clusters containing alkaline-earth metal atoms are measured by a photoionization threshold method to examine the hydration process of the metal atoms in clusters. IPs of Mg(H2O)(n) and Ca(H2O)(n) are found to decrease with increasing n and become constant at 3.18 eV for n >= 9 and n >= 8, respectively. The observed constant IP agrees with an estimated photoelectric threshold (3.2 eV) of bulk ice. From the comparison with the results on the theoretical calculations as well as the IN for alkali atom-water clusters, the anomalous size dependence of IPs is ascribed to the formation of an ion-pair state. (c) 2005 Elsevier B.V. All rights reserved.