Abstract: The effects of gas components such as O₂,SO₂ and HCl,the existence of fly ash,and temperature on mercury speciation were investigated.It is shown that high temperature is positive to mercury oxidation in most of the simulated flue gas system except for the N₂-SO₂ system.The existence of O₂,SO₂ and HCl in the system separately improves the oxidation process,and among them HCl performs the best.When both SO₂ and HCl are in the system,the oxidation efficiency of HCl decreases due to the reaction between SO₂ and HCl.Fly ash provides,reaction media and catalysts for the mercury oxidation reaction,and consequently,the existence of fly ash in the simulated flue gas can increase mercury speciation.