Homogeneous mercury speciation in combustion-generated flue gases was modeled by a detailed kinetic model consisting of 107 reactions and 30 species. This kinetic model includes the oxidation and chlorination of key flue-gas components, as well as six mercury reactions involving HgO with new reaction rate constants calculated neither from experimental data nor by estimated, which was commonly used by other investigators before, but directly from transition state theory (TST). An important and previously unrecognized pathway of homogeneous Hg oxidation mechanism including Hg reactions involving HgO was proposed. Among those reactions involving HgO, the progress of reaction HgO+HCl-->HgCl+OH is HgO+HCl-->TSl(HgClOH)-->M(HgClOH)-->TS2(HgClOH)-->HgCl+OH, in which the controlling step is HgO+HCl-->TSl(HgClOH)-->M(HgClOH). The progress of reaction HgO+HOCl--> HgCl+HO2 is HgO+HOCl-->M(HgClOOH)-->TS(HgClOOH)-->HgCl+HO2, in which the controlling step is M(HgClOOH)-->TS(HgClOOH)-->HgCl+HO2. Four other reactions are one-step, with no intermediates formed. The performance of the model was assessed through comparisons with experimental data conducted by three different groups. The comparison shows that model calculations were in agreement with only one set of all the three groups experimental data. The deviation occurs due to the absence of accurate rate constants of existing mechanism, the adding of reactions involving HgO, as well as the exclusion of heterogeneous Hg oxidation mechanism. Analyses by quantum chemistry and sensitivity simulations illustrated that the pathway Hg+ClO = HgO+Cl is more significant than some of the key reactions in the kinetic mechanism proposed in the literature, which indicates the necessity of including reactions involving HgO in the mercury speciation kinetic mechanism. Studies on the effects of oxygen show that O-2 weakly promotes homogeneous Hg oxidation, especially under the condition of low Cl-2 concentration. In all cases, 1.5-6.0% of the mercury is predicted to be present as HgO. (C) 2003 The Combustion Institute. All rights reserved.