4-R-1,2,4-triazoline-3,5-diones (R = Me (MTAD), R = Ph (PTAD)) react stereospecifically with transcyclooctene (1) to give addition products 2, 3, and 4. The products of the reaction and those obtained from nucleophilic trapping of the intermediate with methanol and water suggest an aziridinium imide followed by an open cation that can lead to transannular ring closure and hydride shifts. At -83 degrees C a trans-aziridinium imide intermediate is formed nearly quantitatively and can be directly observed via NMR spectroscopy. An activation energy of 16.2 kcal/mol was measured for the decomposition of the aziridinium imide. A mechanism is proposed for the reaction.