The bis-2-(trimethylsilyl)ethyl diselenide was prepared and used as a selenating reagent for the insertion of the 2-(trimethylsilyl)ethylseleno group at the C-4 position of the azetidinones. Bis-4-(azetidin-2-one) diselenides were used for the first time as a key intermediate for the synthesis of selenium-containing β-lactams i.e. selenacephams and selenacephems.