H-mordenites are active for the SCR reaction but they suffer irreversible partial deactivation after being on stream for one hour at 650 degrees C. The reaction orders and activation energies are not significantly affected by deactivation. This indicates that deactivation originates in a decrease in the number of active sites due to dealumination and possible pore blockage. The NO disappearance rate correlates with TPD NH3 between 300 and 700 degrees C:; FTIR confirms these results. Xe-129 NMR of adsorbed xenon shows that pore blockage occurs and is due to the presence of aluminum species in the main zeolite channels. The overall deactivation process and the role of acid sites is discussed in terms of the current literature.