Abstract: The effect of arsenic on the performance of commercial V₂O₅-WO₃/TiO₂ catalyst for selective catalytic reduction (SCR) of NO_x was investigated with the assistance of XRD, N₂ sorption, NH₃ chemisorption, FT-IR and XPS; an arsenic poisoning mechanism was proposed. The results illustrated that As₂O₃ in flue gas is a serious toxicant to the V₂O₅-WO₃/TiO₂ catalyst. As₂O₃ adsorbed on the catalyst are mostly oxidized to As₂O₅, which covers the catalyst surface and results in a decrease of surface area and active V sites; as a result, the adsorption of NH₃ is then inhibited, which leads to the deactivation of the V₂O₅-WO₃/TiO₂ catalyst for SCR.