Electrochemical oxidation of Mn2+ with and without the presence of Bi3+ was studied using voltammetric and in situ spectro-electrochemical techniques at boron-doped diamond (BDD) electrodes in 1.0 M HClO4. Electrochemical oxidation of only Mn2+ resulted in the formation of mostly MnO2 with MnO4- produced as a minor product. The MnO2 film formed on the electrode surface, which is an inevitable part of Mn2+ oxidation, shows a blocking effect on the formation of MnO4-, and reduces the overall current efficiency of MnO4- production. Higher Mn2+ concentrations result in less MnO4- production due to the formation of more MnO2. The addition of Bi3+ increased the current efficiency of MnO4- production. The Bi3+ is oxidized to Bi(V), which acts as an electrocatalyst in MnO4- production. The Bi(V) oxidizes MnO2, formed on the electrode surface, to MnO4-. This increases the production of MnO4-by removing the blocking film to provide an active electrode (bare BDD) surface, which is available for further Mn2+ oxidation. (C) 2004 The Electrochemical Society.