During the combustion of coal in power plants Hg is released from the coal into the environment. Several technologies are under developed to reduce these emissions, but the need to implement new control systems will depend in part on the extent to which Hg can be captured in the fly ash. Previous studies have demonstrated that fly ashes may not only retain Hg species but also oxidize Hg-0(g). This should be taken into consideration when developing technologies for Hg retention. The aim of this study is to acquire a better understanding of the interactions between Hg-0(g) and fly ashes and to clarify the effect of unburned carbon and the flue gas composition upon the retention and oxidation mechanisms. A series of retention, oxidation and desorption experiments were carried out using lab-scale reactors. All the results obtained indicate that the interaction between Hg and fly ash is mainly chemical since the retention of Hg involves oxidation. Moreover, if the oxidation takes place in gas phase, condensation of oxidized mercury occurs. Carbonaceous matter is involved in most of the retention and oxidation mechanisms between Hg and fly ash. The carbon concentrates with the highest Hg retention capacity produce the highest Hg oxidation. The gas mixtures containing O-2 + CO2 + SO2 + H2O were observed to increase Hg retention in the carbon concentrates from fly ashes. However, the presence of HCl in the mixtures may increase or decrease Hg capture. Heterogeneous oxidation was only significant in the fly ash fractions enriched in unburned carbon. (C) 2011 Elsevier B.V. All rights reserved.