The deactivation of Ce-zeolite deNO(x) catalysts has been studied. Catalysts were aged in exhaust gases of a diesel engine and a natural gas engine. The deactivation in real exhaust gas has been compared to deactivation in simulated exhaust gas. Ammonia and urea were used as reducing agent. Deactivated catalysts were characterized with amongst others XPS, XRF and DRIFT. Zeolite deNO(x) catalysts show different types of deactivation apart from hydrothermal dealumination. Deactivation may occur at low and at higher temperatures. Several depositions were found on the catalysts: coke, CxHy, S, Zn, P, N and Ca. Coke, CxHy and N containing species can be removed by heating under oxygen rich (deNO(x)) conditions resulting in (partial) recovery of the catalyst activity. The type of lubricating oil used in the internal combustion engine might be an important factor in the deactivation of the deNO(x) catalyst.