High resolution photoemission and first-principles density-functional calculations (DF-GGA) were used to study the chemistry of NO and NO2 on MgO(1 0 0). NO is weakly adsorbed. DF results predict an NO adsorption energy of 6 kcal/mol, close to the experimental value of similar to4 kcal/mol. On MgO1-x, O vacancies enhance the bonding energy of NO and lead to N2O and N production. NO2 is very reactive on MgO(1 0 0) forming NO3 species. The large differences in the reactivity of NO and NO2 reflect the fact that nitrogen dioxide is a good electron acceptor and its orbitals mix better with the occupied bands of MgO.