The nonlinear behavior in the NO-H-2 reaction over Lr(110) was studied with X-ray photoelectron spectroscopy (XPS). At a NO pressure of 7.7 x 10(-8) mbar, three different H-2/NO mixtures were examined in the temperature range from 300 to 750 K, in addition to a mixture with a 10 times higher NO pressure. During the heating-cooling cycle in the reaction mixture, three different N species could be observed-molecular NO, atomic N, and NHx-as well as two O species which could be assigned to molecular NO and atomic O. Simultaneously, the formation rate of N-2, H2O, and NH; was followed. It appeared that under low H-2, pressure conditions no NH3 is formed, and as a result, no NH, could be detected on the surface. The results show that the unreactive surface is covered with atomic O and molecular NO, whereas the active surface is either covered with atomic N (and NH,) or adsorbate free. The presence of atomic O can suppress the NO dissociation up to 600 K, and it has a strongly destabilizing effect on atomic N. Furthermore, the dissociative adsorption of hydrogen can be blocked by a critical N coverage or by the combination of molecular NO and atomic O. On the basis of these results, a model is proposed for the oscillations in activity and selectivity.