This study was mainly focused on the transformation of alkali metals in flue gas, the relationship between mercury oxidation and alkali metals, and the effect of concentrations of Cl-2 and SO2 on mercury oxidation. Thermodynamic stability of alkali metals during coal combustion was analyzed with the chemical thermodynamics software, Chemical Equilibrium with Applications (CEA), and so were the interactions among alkali metals and Cl-species or S-species. The analysis results suggested that mercury oxidation in postcombustion gases could be enhanced by alkali metals. Based on the previous studies, a homogeneous Hg/C/H/O/N/S/Cl/K/Na kinetics model under isothermal conditions was established in this paper, and the kinetics calculation and sensitivity analysis were realized by CHEMKIN 4.1. According to the kinetics model, it is concluded that the main products for K and Na are KSO2, KCl, NaSO2, and NaCl at 793 K Additionally, the impact of alkali metals on mercury oxidation will vary with the concentrations of chlorine and sulfur species in the gas. Cl-2 and SO2 in different concentrations were added for the simulation conditions of this study. Mercury oxidation will be greatly inhibited with the low concentrations of chlorine and SO2, while it will be somewhat promoted with the high concentrations of chlorine and SO2. The sensitivity analysis showed that the alkali metals influenced mercury oxidation mainly by the reactions with Cl-species and S-species, and the mercury oxidation was suppressed when alkali species reacted with Cl-species but promoted when alkali species reacted with S-species, further supporting the kinetics calculation results. However, more work should be done to estimate the mercury oxidation under nonisothermal conditions, not only for homogeneous mercury oxidation but also for heterogeneous mercury oxidation.