The aim of this study was to characterize Pd catalysts supported on three different mordenite zeolites (a commercial sample and two laboratory preparations) previously tested for the NO decomposition reaction. H-2 chemisorption, TPR, FTIR spectroscopy of adsorbed CO and NO, DRS UV-vis and TPD of NO were used to characterize the catalysts. After reduction at 573 K a large fraction of Pd-o particles and some Pd2+ ions were found in the commercial sample and one laboratory preparation. In contrast a sample prepared with a mordenite synthesized using a natural clay was found to exhibit mainly ionic Pd. The amount of Pd-o (PdO) relative to Pd2+ depends on the oxidation temperature and the metal loading. The catalysts exhibited similar levels of activity for the NO decomposition reaction at 673 K but a different selectivity to N2O. This behavior was related to differences in metal dispersion and other surface conditions that influence the formation of PdO particles.