The process of the reaction among elemental mercury (Hg-0) and reactive flue gas components across the selective catalytic reduction (SCR) catalyst was studied in a laboratory-scale reactor. Prepared vanadia-based SCR catalysts were characterized and analyzed to understand the potential reaction pathways. Mercury oxidation was observed when pro-exposure of the SCR catalyst to HCI, followed by passing through Hg-0/N-2 in the absence of gas-phase HCI. At testing conditions, Hg-0 was found to desorb from the catalyst surface by adding HCI to the gas steam, which implies that HCI adsorption onto the SCR catalyst is strong relative to the mercury. Surface analysis verified the absorption of HCI onto the SCR catalysts, and the potential reaction pathways were proposed. Indeed, the monomeric vanadyl sites on the catalyst surface were found to be responsible for the adsorption of both Hg-0 and HCI, which means they are active for mercury oxidation. Furthermore, the detailed Langmuir-Hinshelwood mechanism was proposed to explain the mercury oxidation on the SCR catalyst, where reactive Cl generated from adsorbed HCI reacts with adjacent Hg-0.