Abstract Experiments and density functional theory calculations were conducted to uncover the reaction chemistry of Hg0 oxidation during SO2/SO3 conversion over V2O5/TiO2 catalyst. The results show that SO2 promotes Hg0 oxidation over V2O5/TiO2 catalyst with the assistance of oxygen. The promotional effect is dependent on the reaction temperature, and is associated with the bimolecular reaction between Hg0 and SO3 over V2O5/TiO2 catalyst. SO2 can be oxidized to SO3 which has high oxidation ability for Hg0 oxidation. SO2/SO3 conversion proceeds through a three-step reaction process in the sequence of SO2 adsorption → SO2 oxidation → SO3 desorption. SO2 oxidation presents an activation energy barrier of 223.84 kJ/mol. HgSO4 species is formed from the bimolecular reaction between Hg0 and SO3 over V2O5/TiO2 catalyst. Hg0 oxidation by SO3 over V2O5/TiO2 catalyst occurs through three reaction pathways, which are energetically favorable for HgSO4 formation. SO2* → SO3* is identified as the rate-determining step of HgSO4 formation. During Hg0 oxidation by SO3 over V2O5/TiO2 catalyst, HgSO4 desorption is a highly endothermic reaction process and requires a higher external energy. The proposed skeletal reaction network can be used to well understand the reaction mechanism of Hg0 oxidation during SO2/SO3 conversion over V2O5/TiO2 catalyst.