The interaction of NO molecules with the surface of MgO has been studied by EPR as a function of outgassing temperature. Two different types of paramagnetic species are identified by EPR: physisorbed No and chemisorbed NO22-. They are formed upon specific adsorption of NO on Lewis acidic Mg2+ and basic O2- sites, respectively. The concentration of NO and NO22- radicals depends on the outgassing temperature of MgO reaching a maximum at 1073 K. FT-IR measurements suggest that sites liberated upon thermal outgassing by removal of OH and carbonate groups adsorbed on MgO play a significant role in the increase of the concentrations of NO and NO22- radicals. The latter are possibly formed on low coordinated oxide ions close to cationic vacancies or O-4C(2-) of monoatomic step and O-3C(2-) from kinks and divacancies. (c) 2008 Published by Elsevier B.V.