A density functional mPW1PW91 study of the most stable conformers of MOH(H2O)(0-3) clusters (M = alkali or alkaline-earth atom) was performed, along with quantum chemistry CCSD(T,Full) calculations for M = Li and Be. The resulting binding energies for hydration, atom-charge and valence-electron distributions allow for a rationalization of the opposite trends in the cluster-size dependence of adiabatic/vertical ionization energies for the alkali and the alkaline-earth monohydroxides. The closed- and open-shell natures of the latter, respectively, determines their behavior during initial steps of their hydration processes, whereas the ionic/covalent character of their M-OH bonds plays a minor role except for BeOH. (C) 2014 Elsevier B.V. All rights reserved.