The two main gaseous mercury species in combustion flue gas are elemental mercury (Hg-0) and oxidized mercury (Hg2+). In the coal combustion process using an installed wet FGD system, Hg2+ is captured and Hg-0 emitted since Hg2+ is soluble in water. Oxidation of Hg-0 upstream of the FGD would significantly reduce Hg emission. In the presence of CuO nanoparticles, oxidation of Hg-0 to Hg2+ proceeded under low HCl concentration in a simulated combustion flue gas. This paper describes the effects of reaction temperature, catalyst particle size, area velocity on mercury oxidation with CuO particles and the performance demonstration of CuO mercury oxidation with actual combustion flue gases. In simulated combustion flue gases, the conversion of Hg-0 to Hg2+ increased as the reaction temperature decreased, but the conversion decreased as the particle size increased. Mercury oxidation with CuO nanoparticles of 50 nm in diameter was not significantly affected by area velocity in the range of 47 to 235 m(3)/(h m(2)), but the mercury oxidation with CuO particles of 1.1 mu m was decreased over 100 m(3)/(h m(2)). In an actual combustion flue gas, CuO nanoparticles showed mercury oxidation as in the case with the simulated coal combustion flue gas. (C) 2007 Elsevier B.V. All rights reserved.