We investigate the interactions of one and two H2O molecules at three-coordinated sites of (MgO)(4) and (MgO)(8) clusters. Energies of adsorption are calculated and compared to similar work involving periodic and embedded cluster systems. The cluster model results predict that H2O chemidissociates without a barrier directly onto adjacent three-coordinated sites. The chemidissociation energy per water molecule is observed to be greater upon the dissociation of two neighboring water molecules compared to the dissociation of a single water molecule : a positive cooperativity effect. Correlation energy treated at the MP2 level of theory reduces the predicted chemidissociation energy by approximately 10 kcal/mol, and it is found that the MP2 single-point energies evaluated at the RHF-optimized geometries, MP2/RHF, differ from the energy of the MP2-optimized structures by approximately 1 kcal/mol.