Abstract: The reaction mechanism for the formation of As₂O₃ by the homogeneous reaction of O₂ with As and AsO in the coal-fired flue gas was investigated by the quantum chemical density functional theory. The structure and energy of each reactant, intermediate, transition state and product were determined and the thermodynamic and kinetic analysis was carried out to explore the reaction mechanism. The results show that the maximum reaction energy barriers for the formation of As₂O₃ from As and AsO are 32.9 and 157.2 kJ/mol, respectively. The forward and reverse reaction coefficients all increase with an increase of the reaction temperature in the range of 500-1900 K, although the influence extent of temperature varies with different reactions. For the oxidation of As, the equilibrium constants of two reactions are always greater than 10⁵, indicating that the oxidation of As can be carried out completely and regarded as an irreversible reaction. In contrast, for the oxidation of AsO, the equilibrium constants are always less than 10⁵, indicating that the oxidation of AsO is an incomplete reaction. The equilibrium constant of the As₂O₃(D3H) configuration is extremely low; however, the formation of the As₂O₃(GAUCHE) configuration is a spontaneous process.