The effect of AI(OH)(2)(+) EFAL (extra-framework aluminum) species in the acid strength of USY zeolite was studied by the ONIOM scheme, at PBE1PBE/6-31G(d,p) level. The model consists of a hexagonal prism and the sodalite cage (T-30), representing a real part of the zeolite Y system that allows calculation of different acid sites (O-1 and O-3). The acid strength was calculated by computing the deprotonating energy and comparing the results with the data obtained for an isolated acid site. The calculations indicated that there occurs an increase in acid strength, depending on the relative position between the EFAL and the acid site. However, the role of AI(OH)(2)(+) EFAL is not to increase the energy of the acid form, through Bronsted/Lewis synergism, but to stabilize the conjugated base, formed upon deprotonation, by hydrogen bonding and nucleophilic interaction with the framework oxygen atom. (c) 2007 Elsevier B.V. All rights reserved.