High concentrations of H2O2 cause inactivation of manganese peroxidase (MnP). The mechanism of this inactivation has been investigated. A method for stabilization of MnP used for pulp bleaching in the presence of high concentrations of H2O2 has been developed. It was demonstrated that formation of the inactive form of MnP (Compound III) is the major reason for the inactivation in which hydroxyl free radicals or superoxide anion radicals are not involved. It was also found that 1-hydroxybenzotriazole, 2,2,6,6-tetramethyl-1-piperidinyloxy free radical, violuric acid, 3-hydroxy-1,2,3-benzotriazin-4-(3H)-one and chlorpromazine stabilized MnP. The stabilizing effect by 1-hydroxybenzotriazole is due to the conversion of the inactive Compound III to the native enzyme.