The hydrophilic property of VPI-5 was investigated using cluster models by an ab initio MO method. The four cluster models of Al(OH)4 studied differ from each other in the original locations of the aluminum atom and in whether they are for the hydrate or the dehydrate. Only one model, which has an aluminum site at the center of the fused 4-membered rings in the hydrated VPI-5, can adsorb a water molecule with an attractive interaction energy of over 25 kcal/mol. The other models show only repulsive interaction energies. The origin of the hydrophilicity was investigated by an energy decomposition analysis. The analysis concluded that (1) for three of the four kinds of Al sites, oxygen atoms around the Al atom prevent any water molecule from coming to the Al site by a large exchange interaction, and (2) for one kind of the Al site, electrostatic and charge transfer terms between the oxygen atom of the water molecule and the Al atom are stronger than the exchange interaction mentioned above. Consequently, the Al site works well as a Lewis acid site. © 1996 American Chemical Society.