The nature of adsorbed nitrogen oxide complexes (NOy) has been identified by FTIR spectroscopy. Nitrito groups are most prominent on Co/ZSM-5. These complexes are stable at 150 degrees C in He but are reduced upon exposure to either C3H8 or CH4 leading to the formation of N-2. Conversely, Cu/ZSM-5 forms nitro and nitrate groups which are stable at 200 degrees C. At this temperature these complexes are chemically reduced to N-2 upon exposure to C3H8 yet remain inert to CH4. At higher temperatures, thermal decomposition yields NO/NO2, O-2, and H2O. The specific rate of NO reduction over Cu/ZSM-5 decreases with Cu loading indicating that isolated Cu2+ . NO2 complexes pay a significant role in the mechanism. The hydrocarbon specificity of these complexes thus matches the known catalytic specificity of NOx reduction over Cu/ZSM-5 and Co/ZSM-5. The mechanistic cause for the catalytic specificity of the ZSM-5 hosted metals in NOx reduction thus is traced back to the structure and reactivity of the specific NOy adsorption complexes which these metals produce upon interacting with NO + O-2. The consumption of CH4 when feeds containing NO, CH4, and O-2 are exposed to Cu/ZSM-5 can be attributed to nonselective combustion with adsorbed oxygen and not to reactions with NOy.