Abstract: The oxidation efficiency of elemental mercury (Hg⁰) by fresh and 40 000 h SCR (Selective Catalytic Reduction) catalysts was studied with the help of simulated experiments under oxidation and SCR conditions, respectively. The structural and textural properties of the catalysts were characterized by various techniques, including N₂ adsorption/desorption, SEM-EDS, FT-IR, and ion chromatography. It was found that the Hg⁰ oxidation efficiency over the 40 000 h catalyst is about 5%~20% lower than that of the fresh one under the oxidation condition (with HCl and O₂), whereas it just decreases by about 5%~10% under the SCR condition (when NH₃ and NO are added). The particles agglomeration phenomenon was observed for the 40 000 h catalyst, of which the BET, V content, and V⁵⁺ = O quantity decrease to some extent. The contents of the water-soluble ions (such as Na⁺, K⁺, NH₄⁺, and SO₄^2-) of the 40 000 h catalyst are higher than those of the fresh one. All these factors have a negative influence on the pore structure and active sites distribution over the 40 000 h catalyst, thereby reducing its Hg⁰ oxidation efficiency.