The mechanisms of the electrochemical oxidation of mono- and dinitroylides of S and Se on a Pt electrode in CH3CN have been studied for the first time. It was shown that the cation radical formed as intermediate was able to react with the medium with removal of an H atom and with the formation of an onium cation or to undergo decomposition. Competition of these processes determines the special features of the electrooxidation of mononitroylides, while on oxidation of dinitroylides the intermediate cation radical decomposed to oxides of nitrogen.