The durability of CuZSM-5 in the selective reduction of NO has received scant attention notwithstanding the prospective use at high temperatures in streams containing high levels of steam, i.e., conditions that cause zeolite dealumination. Dealumination is synonymous with the loss of Bronsted acid sites and with the destruction of the geometric integrity of the exchanged Cu-ion sites. If either of these sites is important in NO reduction, dealumination will result in deactivation. Another issue is the effect of copper exchange on dealumination. We have followed the decrease in the tetrahedrally coordinated27 Al MAS n.m.r. peak at 54 ppm to assess the extent of the dealumination. The treatment of 43%-exchanged CuZSM-5 (SiO2Al2O3 = 30) in a static system at 700°C established that Cu inhibits the dealumination. For a different CuZSM-5 with a high SiO2Al2O3 ratio of 80 and “overexchanged” at 147%, it was observed that the activity decreases rapidly at 410°C if the catalyst is exposed to steam-containing air, whereas it is not affected by dry air. The decrease in activity is accompanied by dealumination. The activity decreased to about half its initial value and remained constant thereafter. The extent of dealumination was somewhat lower, proportionately, than the loss of activity. Some inferences are drawn from the experimental results.