The effect of halogen acids HCl, HBr and HI on mercury conversion was studied in a laboratory-scale SCR reactor using simulated flue gases, and is presented here, Two types of commercially available SCR catalysts, Honeycomb and Plate type catalysts, were used in these studies. HBr and Hi both had shown much stronger effects on mercury conversion than HCl. Both HBr and HI oxidized more than 85% of the gas phase mercury at a low concentration of 2 ppm. The age of the catalyst and the type of catalyst also have an effect on mercury conversion. A larger extent of mercury oxidation was observed in the presence of a Honeycomb catalyst than with the Plate catalyst. (C) 2008 Elsevier B.V. All rights reserved.