Pd was introduced by exchange in non-dealuminated and dealuminated mordenites. After activation under oxygen at 500 degrees C, NO adsorption revealed the presence of isolated Pd2+ ions and Pd/PdO particles. Two types of Pd ions were shown : the first one would be located in the small cavities and leads to the formation of dinitrosyl complexes, whereas the second one, probably sitting in the main channels, is responsible for the obtainment of mononitrosyl complexes. Pd/PdO particles are active in the direct combustion of CH4 by oxygen and poorly active in the selective reduction (SCR) of NO. Pd ions are active for the NO SCR; the ions in the main channels are thought to be more active than the ions in the small cavities for accessibility reasons. In the case of the non-dealuminated mordenite catalyst, an activation under reactants is observed. Moreover, the activity is increased by the addition of 10 vol% water which is the opposite of the classical behavior of metal-loaded zeolites. Both phenomena are attributed to a migration of Pd ions from hidden to accessible sites. For the dealuminated sample, the observed deactivation seems to be due to a migration of isolated Pd ions into PdO particles located outside the zeolite matrix.