Electrochemical oxidation of Mn 2ϩ with and without the presence of Bi 3ϩ was studied using voltammetric and in situ spectroelectrochemical techniques at boron-doped diamond ͑BDD͒ electrodes in 1.0 M HClO 4 . Electrochemical oxidation of only Mn 2ϩ resulted in the formation of mostly MnO 2 with MnO 4Ϫ produced as a minor product. The MnO 2 film formed on the electrode surface, which is an inevitable part of Mn 2ϩ oxidation, shows a blocking effect on the formation of MnO 4 Ϫ , and reduces the overall current efficiency of MnO 4 Ϫ production. Higher Mn 2ϩ concentrations result in less MnO 4 Ϫ production due to the formation of more MnO 2 . The addition of Bi 3ϩ increased the current efficiency of MnO 4 Ϫ production. The Bi 3ϩ is oxidized to Bi͑V͒, which acts as an electrocatalyst in MnO 4 Ϫ production. The Bi͑V͒ oxidizes MnO 2 , formed on the electrode surface, to MnO 4 Ϫ . This increases the production of MnO 4 Ϫ by removing the blocking film to provide an active electrode ͑bare BDD͒ surface, which is available for further Mn 2ϩ oxidation.