In recent years, the alkali metal-induced deactivation of V2O5/TiO2 catalysts during the selective catalytic reduction (SCR) of NO by NH3 has received much attention. The presence of SO2 in flue gas may influence the alkali metal-induced deactivation because of its possible interactions with catalysts and alkali metals. This study aims to evaluate the combined effect of SO2 and alkali metals on the SCR activity of V2O5/TiO2 catalysts and identify the compound responsible for catalyst deactivation. KCl, the typical alkali substance in flue gases, was doped to in-house prepared V2O5/TiO2 catalysts by impregnation, and the KCl to V2O5/TiO2 (K/V) molar ratio was controlled at 0-0.3. The catalysts were subjected to the activity test for >120 h and characterized by BET, XPS, and FTIR. The results indicate that SO2 increases the deactivation of V2O5/TiO2 catalysts by KCl, and the increased extent depends on the amount of KCl deposited on the catalyst. At a K/V molar ratio of 0.1, K2S2O7 was formed and interacted with V2O5 to produce a eutectic; however, at a K/V ratio of 0.3, the eutectic was not observed. This eutectic significantly decreased NH3 adsorption, particularly those on Bronsted acid sites, and weakened the oxidation ability of the catalyst, thus significantly decreasing the SCR activity. (C) 2014 Elsevier B.V. All rights reserved.