A small amount of manganese compound contained in limestone as an impurity takes an important role of a catalyst for the sulfite oxidation reaction. In the present work, it was found that Mull coprecipitates with the calcium sulfite solid in a continuously-stirred tank reactor under the similar conditions as that in the absorber tank of the wet limestone scrubbing process. The experimental results indicated that the coprecipitaton ratio of Mn2+ to calcium sulfite solid increases linearly proportional to the dissolved Mull concentration in the solution upto a critical Mn2+ concentration and remains constant in the range of higher Mn2+. It was also found that Mn2+ did not coprecipitate with gypsum crystal at all.