Two types of NH4+ cations can he distinguished in the surface layer of NH: mordenite by XPS, viz., NH4+ cations coordinated in the side pockets (22%) and in the main channels (78%). Additionally, nonframework SiOx species were observed in the mordenite. Thermal activation of NH4+ mordenite at 400 degreesC causes the appearance of two types of atomic nitrogen strongly chemisorbed on these SiOx nonframework species.