In the present work. we report theoretical calculations of the interaction of H2O, SO2, O-2, NO2 and NO with copper-exchanged zeolite (Cu-ZSM5). Calculations were carried out using ab initio density functional theory (DFT). The DF calculations show that the adsorption energies of NO2 and NO are larger than the corresponding adsorption energies of water and oxygen. Therefore, neither water nor O-2 can easily displace the adsorbed NO(x)s. On the contrary, water can displace adsorbed O-2 over the Cu, which indicates that the water could blocks some reaction channels of the catalytic cycle. It was also found, that the adsorption of SO2 is not strong but can obstructs completely the active site for the reduction of the NOx due to the formation of a CUSO4 specie. The topological analysis of the molecular charge density shows that the O-2 and SO2 molecules are activated after the adsorption over the Cu, whereas H2O is not activated. (C) 2004 Published by Elsevier B.V.