In this study, we carried out two-layer ONIOM calculations to determine the Gibbs free energy and the enthalpy changes for the gallium-exchanged silicoaluminophospate (Ga/SAPO-11) catalysts' interaction with NH3, SO2, CH3NH2, CH3SH, H-2, and H2O molecules, and the formation of possible intermediate species, such as oxy-hydroxy GaOm (OH)(n). The results reveal that the [HGaOH](+) and [Ga](+) formation, as well as the SO2 adsorption at 773 K are thermodynamically favorable processes. This work shows for first time the potential ability of the Ga/SAPO-11 catalyst to adsorb SO2 and to oxidize H2S; that is, to be used in DeSOx reactions. NH3 and CH3NH2 adsorption were also found to be exothermic processes, but thermodynamically favourable (Delta G < 0) at temperatures below 773 K, whereas CH3SH is not adsorbed at all. This implies that the Ga/SAPO-11 catalyst behaves like a hard acid according to Pearson's rule. (C) 2007 Elsevier B.V. All rights reserved.