Manganese is the most effective catalyst for the hydrogen peroxide decomposition during peroxide bleaching. used in the pulp-and paper industry., We studied the chemistry involved in this project. The results showed that Mn2+ and Mn3+ are present during the course of the reaction. We followed the Mn2+ and Mn3+ concentrations during the course. It was found that a steady state of Mn3+ concentration was attained until the hydrogen peroxide concentration was zero. At this time, the Mn3+ concentration was maximum, then it decreased due to the disproportionation to form MnO2. Removal of O-2 from the system slowed down the rate of decomposition. The detailed reactions involved were discussed.