Quantum-chemical calculations were performed using the DFT method and a qualitative parametric quantum method (PQM) named CATIVIC, in order to model the whole EFAL formation process. Two clusters (AlSi3O12H9 and AlSi63O152H49) were employed to model zeolite ZSM-5. Formation of intermediate Al species was reported with four to zero bond coordinations to the zeolite framework. The EFAL formation process was analyzed by the evaluation of bond distances, diatomic bond energies changes (Delta DBE) and Wiberg's indexes (WI) of Al-O bonds. Penta- and hexacoordinated intermediates were produced and free Al(OH)(3) and Al(OH)(3) (H2O)(2) EFAL species were formed from the small and big clusters, respectively. Comparisons with the DFT method permit establishing that CATIVIC or a well-parameterized PQM method can be used to understand zeolite changes under hydrothermal treatment. (C) 2008 Elsevier B.V. All rights reserved.