The thermal rearrangement of 2,4,6-trimethylbenzonitrile N-oxide to the corresponding isocyanate has been investigated by double isotopic (2H and 18O) labelling and kinetic experiments. Rearrangement proceeds with exchange of oxygen atoms, and this exchange occurs exclusively between nitrile oxide molecules. Kinetic results suggest that the pathway involves a polymerisation, catalysed either by electrophiles or by nucleophiles, followed by decomposition of the polymer. The rearrangement is catalysed at room temperature by trifluoracetic acid. A synthesis of 18O-labelled hydroxylamine is reported.