Pentafluoro- and pentachloro-benzonitrile N-oxide were prepared from the corresponding pentahalogenobenzaldehyde oximes. The fluoro-compound dimerised to 3,4-bis(pentafluorophenyl)furoxan, whereas the chlorocompound was stable. Both the nitrile oxides readily underwent cycloadditions with styrene and phenylacetylene, to give the appropriate 3-pentahalogenophenyl-5-phenyl-isoxazolines and -isoxazoles.