Spin-forbidden N-O dissociation has been studied for N2O, [Cu-ON2](+), and ZCu-ON2. For N2O and [Cu-ON2](+) the minimum energy points of singlet-triplet surface crossing have been located. Moreover, the energy necessary to reach the crossing points has been estimated from the energy profiles of singlet and triplet states at different values of the N-O distance. The comparison of the results obtained for N2O, [Cu-ON2](+) and ZCu-ON2 shows that the catalytic effect of the zeolite can be directly related to the strength of the Cu-O bond in ZCuO. (C) 2002 Elsevier Science B.V. All rights reserved.