The co-oxidation of elemental mercury (Hg) in SCR DeNOx reactors to water soluble oxidised species followed by co-removal in flue gas desulphurisation scrubbers is a promising way to reduce these emissions from fossil fuel fired power plants. This paper looks at the results of a series of lab experiments on the oxidation and reduction of Hg by commercial V2O5/WO3(MoO3)/TiO2 honeycomb and plate type DeNOx catalysts. It could be shown that at a given hydrogen halide content the oxidation of elemental Hg on DeNOx inactive material follows a first order reaction with respect to the Hg concentration. On the basis of mass concentration, HBr was ten times more effective than HCI in promoting the oxidation of elemental Hg under otherwise identical conditions. The oxidation of elemental Hg increased with increasing vanadium content in the catalysts. The Hg oxidation activities of several commercial SCR catalysts were determined for the DeNOx inactive case and compared with their DeNOx activities. In this way, it was possible to show that the velocities of Hg oxidation and DeNOx reaction are of the same order of magnitude. The DeNOx-active state of the catalysts had a strongly negative impact on the oxidation of elemental Hg that could not be explained solely by an inhibition effect of ammonia. Under these conditions, even reduction of oxidised Hg occurred on the catalyst. The same reduction occurred for SCR DeNOx catalysts when oxidising volatile organic hydrocarbons. This effect was called induced mercury reduction. The observed oxidation rate of Hg on DeNOx- and VOC-active catalysts is the net sum of an oxidation and a reduction reaction which take place in parallel. Our understanding of the control variables for Hg oxidation in SCR DeNOx plants has to be amended in light of these slowing effects and has to take the induced reduction into account. (C) 2013 Elsevier B.V. All rights reserved.