In this paper, we review the reactivity of the paramagnetic NO molecule with Ni-doped MgO based on electron paramagnetic resonance (EPR) spectra and density functional theory (DFT) cluster model calculations. The EPR spectrum of the NiO/MgO sample recorded at 77 K under NO pressure is characterized by two distinct signals, one assigned to monomeric NO physisorbed on low-coordinated Mg2+ ions and the other to NO adsorbed on terrace Ni2+ ions. The latter interaction is found to be at least three times larger than the former. The nature of NO bonding to Ni2+ ions is analyzed in detail. The EPR spectrum proves the presence of one unpaired electron localized on the Ni 3d shell. The DFT calculations indicate that the impaired electron sits on the d(x2-y2) orbital, while the Ni2+ d(z2) electron is coupled with the NO pi(xz)(*) unpaired electron, producing a NO-Ni2+ bond with dominant covalent character. (C) 2003 Elsevier B.V. All rights reserved.