The electrochemical oxidation of selenanthrene was studied by conventional and microelectrode techniques. The first oxidation signal was proved to follow an ECE mechanism : steady-state voltammetry at microelectrodes and cyclic voltammetry showed a monoelectronic transformation into the cation-radical, whereas controlled potential coulometry and electrolysis gave the corresponding 5-selenoxide with an exchange of 2 F mol-1. The electrochemical reversibility of the intermediary cation-radical was investigated and its stability compared with those of similar heterocycles. Further oxidation led to 5,10-selendioxide.