Results are summarized for a study on the effects of poisons on the La-Fe-ZSM-5 catalyst activity for the selective catalytic reduction of NO by ammonia. The deactivation of La-Fe-ZSM-5 honeycombs was studied in field tests. A honeycomb catalyst containing 25%La-Fe-ZSM-5 had an overall activity similar to that of a commercial vanadia honeycomb catalyst. Long-term activity test results show that the 25%La-Fe-ZSM-5 catalyst activity decreased to 50% after 300 h and 25% after 1769 h of on-stream flue gas exposure. The deactivation is correlated to the amounts of poisons deposited on the catalyst. Poisons include alkali and alkaline earth metals, As and Hg. Hg was found to be ion-exchanged from HgCl2 to form Hg-ZSM-5, and Hg was found to be among the strongest poisons. The poisoning effects of these elements appeared to be additive. Thus, from the chemical analysis of the deactivated catalyst, the deactivation of Fe-ZSM-5 can be predicted. (C) 2004 Elsevier B.V. All fights reserved.