Although it has been already reported that manganese plays an important role in the oxidation of sulfite ion, the influence has not yet been understood exactly and the oxidation rates reported in the existing investigations are much lower than those estimated from the performance of actual FGD systems. Manganese compounds are usually contained in raw limestone as the trace element, and concentration of Mn2+ in the absorption slurry is of several ppm. It was found in this investigation that Mn2+ ion is inevitable for the oxidation of Ca(HSO3)(2), the oxidation rate in the homogeneous liquid phase is proportional to the [Mn2+](2) and 10((0.1038pH)), and the oxidation rate of Ca(HSO3)(2) is about 16 times as high as that of H2SO3. It is further predicted that the limited absorption rate of oxygen into the absorption calcium slurry makes the oxidation rate appear linearly dependent on the partial pressure of oxygen in the flue gas and does not depend on Mn2+ concentration as the Mn2+ concentration is so high that the chemical reaction rate is larger than the oxygen absorption rate into the slurry. The result agrees well with practical experience with FGD systems.