The structure and stability of nitrosyl complexes formed in Cu-FER zeolite were investigated using a periodic DFT model. The reliability of both DFT methods and cluster models when describing the Cu(+) interaction with NO molecules was examined. The relative stabilities of mononitrosyl complexes on various Cu(+) sites in Cu-FER are governed by the deformation energy of the particular site. Three types of dinitrosyl complexes with different coordination on the Cu(+) cation were identified: (i) four-fold tetrahedral, (ii) four-fold square-planar and (iii) three-fold trigonal-planar complexes. The most stable dinitrosyl complex, formed when the two NO molecules interact with Cu(+)via the N atom, has a tetrahedral coordination on Cu(+). The cyclic adsorption complex, having a square-planar arrangement of ligands on Cu(+) and interaction via O atoms, is only about 10 kJ mol(-1) less stable than the N-down dinitrosyl complex. This cyclic dinitrosyl complex is suggested to be the key intermediate in the deNO(x) process taking place in Cu-zeolites.