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Arsenic (As) is found to be poisonous to the commercial V 2 O 5 WO 3 /TiO 2 catalysts for selective catalytic reduction of NO x with NH 3 . The NO x conversion of catalysts declines and N 2 O formation dominates at high temperature (above 300 °C) after As poisoning. A series of activity and characterization experiments are applied to reveal the deactivation mechanism caused by As. Results indicate that doping of As on the catalysts, which exists as H 2 AsO 4 − and HAsO 4 2− , doesn’t seriously change surface area of catalysts or TiO 2 phase, but greatly decreases both the Lewis and Brønsted acid sites. It is found that V OH is destroyed and less reactive As OH is newly formed. V OH site is crucial to the NH 3 adsorption and its destruction by As contributes to catalyst deactivation. Besides, stronger oxidizability of catalysts that resulted from more surface-active oxygen aroused by As leads to substantial non-selective catalytic reduction reaction and NH 3 oxidation at high temperature. Both of these two aspects result in lower NO x conversion and higher N 2 O formation. However, SO 4 2− can provide remarkable surface acidities and the sites that destroyed by As can thus be supplied. Benefited from these new reactive acid sites, catalysts with prominent SO 4 2− loading show superior arsenic resistance even with a high As content, which indicate to be a promising anti-poisoning formula. Finally, mechanism of arsenic poisoning and resistance effect of SO 4 2− is proposed based on the above analysis. [ABSTRACT FROM AUTHOR]