The oxidation of diphenyldiselenide with ammonium peroxydisulphate is a very simple and efficient method to produce phenylselenium cations in the absence of nucleophilic counter ions. The reaction carried out in the presence of an olefin, in acetonitrile or propionitrile containing trifluoromethanesulphonic acid and water afforded the amidoselenenylation products in good yield. The intramolecular