The hydrothermal stability of Ce3+ zeolite catalysts used for selective catalytic reduction of NOx was investigated. Aging of Ce-ZSM-5, Ce-beta, Ce-mordenite and Ce-Y catalysts consisted of steaming in 10 or 12 vol% water at 600 degrees C for 3-99 h. Ce-ZSM-5 (Si/Al ratios: Si/Al=17.1, 22.6 and 146.6) and Ce-mordenite (Si/Al=6.4, IE=77.2%) showed fast deactivation. Ce-beta (Si/Al=12, IE=68.4%) and Ce-Y (Si/Al=2.8, IE=122%) are significantly more stable zeolite catalysts, Ce-beta being the most active of these two. Ce-beta and Ce-ZSM-5 catalysts - both having high initial activities - were characterized with Si-29-NMR and Al-27-NMR. Especially Ce-ZSM-5 showed an increase of non-framework Al, meaning that the zeolite suffered from dealumination.