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• Thallium (Tl) poisoning phenomena on used SCR catalysts in China is first reported. • Tl 2 O 3 and Tl 2 SO 4 on the catalyst surface covers the Lewis and Brønsted acid sites. • Consumption of the surface acid sites causing severely catalyst deactivation. • An effective regeneration method for Tl poisoned SCR catalysts has been developed. Thallium (Tl) poisoning is reported for the first time occurring on selective catalytic reduction (SCR) catalysts running in a certain cement kiln. NO x conversion of the poisoned catalyst is almost lost with 6.4 % Tl loading even at 400 °C. Besides the decrease of the BET surface area, Tl significantly decreases the oxidation ability of the active sites due to the coverage effect. Tl is present as both Tl 2 O 3 and Tl 2 SO 4 and covers both the Lewis and Brønsted acid sites of V 2 O 5 and the Ti (OH) Ti bridging sites of TiO 2 , leading to a marked decrease in the quantity and strength of acid sites. Similar to the regeneration of alkali-poisoning catalysts, dilute H 2 SO 4 is also effective to remove partial surface Tl 2 O 3 , by which yielded 89 % of Tl removal with a 0.4 M H 2 SO 4. However, the SCR performance could not total recover because of the remnant Tl 2 O 3 on the catalyst regenerated only once. [ABSTRACT FROM AUTHOR]