In this paper, we present quantum chemical calculations for copper sites in the extended model of ZSM-5 framework based on seven T sites in two fused 5T rings cut off the MFI structure. Geometrical and electronic properties of cationic copper centres and their adsorption complexes with NO molecule are analysed from the point of view of the activation mechanism. The calculations show that the NO molecule adsorbed on Cu+ site becomes significantly activated contrary to Cu2+ centre, in full accordance to IR measurements. Our results indicate clearly that the ability of Cu+ site to donate electrons to pi-antibonding orbital of NO causes significant bond weakening and makes Cu(+)ZSM-5 active in decomposition of nitrogen oxides.