Emissions of gaseous mercury from combustion sources and their control are a major environmental concern facing power generators. The removal of mercury by conversion to mercury halides is evaluated by use of an elementary reaction mechanism that is developed from fundamental principles of thermodynamics and statistical mechanics. Thermochemical properties have been calculated for needed reaction intermediates using CBS-QB3 and density functional methods, and kinetics are from evaluated literature and calculations. An elementary reaction mechanism has been constructed with quantum Rice-Ramsperger-Kassel analysis for chemical activation, and dissociation reactions with Master Equation for fall off. Comparisons of mercury loss versus halogen, hydrocarbon, H2, H2O, O2, CH4, and NOx presence in a typical combustion effluent stream are performed. Results illustrate significant effects of H2 on the formation of HgCl2, and competing effects of NOx species with Hg for the halides. [Supplementary materials are available for this article. Go to the publisher's online edition of Combustion Science and Technology for the following free supplemental resource: a listing of rate constants at different pressures for association, addition, and insertion (chemical activation) and dissociation reactions for mercury species, plus lists of thermochemical properties and rate constants for each detailed mechanism subreaction set.].