Density functional and ONIOM calculations were carried out to investigate the possible active sites of Au(I) ion-exchanged ZSM-5 catalysts. Adsorption energies and vibrational frequencies of CO, NO, NO2, SO2, H2O, NH3, CH3NH2 and CH3SH molecules were analyzed. For first time the existence of two active sites in Au/ZSM-5 is shown; one of them is responsible for the high and the other for the low CO and NO adsorption frequencies. The analysis of adsorption Delta G shows that Au(I) behaves like a soft acid; therefore, there will be a preferential adsorption of soft bases over hard bases. The interaction with NH3 is thermodynamically very favored, mainly Clue to the hydrogen bond formation between the NH3 hydrogen atoms and the oxygen atoms of the zeolitic framework. (C) 2008 Elsevier B.V. All rights reserved.