A site-poisoning and -promotion technique was employed to investigate the role of adsorbates and to formulate a reaction pathway for the NO decomposition reaction over Cu-ZSM-5. Transient infrared and mass spectrometer studies of pulse NO reaction on under- and over-exchanged Cu-ZSM-5 reveal Cu2+(NO), Cu+(NO), bridging Cu2+(NO3-), and NO+ as the major adsorbates and N-2, N2O, O-2, and NO2 as the products. SiH4 and H2O moderately inhibited Cu+(NO) and N-2 formation, but severely inhibited CU2+(NO3-) and O-2 formation. Addition of CO as a reducing agent led to the promotion of Cu2+ reduction to Cu+, depletion of Cu2+(NO3-), increase in NO conversion and O-2 formation as well as formation of CO2. These results revealed that N-2 and O-2 formation proceeds through separate rate-limiting steps; O-2 formation occurs via both autoreduction of CU2+ to Cu+ and decomposition of Cu2+(NO3-). The results from pulse reaction studies on deactivated Cu-ZSIW-5 reveal that O-2 formation is more sensitive to poisoning than NO dissociation and N-2 formation. Although silanation may improve the hydrothermal stability of the catalyst, silanation severely inhibited O-2 and Cu2+(NO3-) formation through inhibition of adsorbed O migration. Promotion of O-2 desorption by addition of a small amount of reducing agent such as CO in the presence of NO (NO/CO > 3.5) can greatly enhance NO decomposition activity. Further investigation is needed to study such an effect on the NO decomposition activity under the O-2 environment.