The title reduction of nitroaromatics ArNO₂ by vinyl halide radical cations CH₂CH–X⁺˙ (X = Cl or Br) to form arylnitrenium ions ArNH⁺ involves a change in oxidation number of nitrogen from +3 to −1. This novel reaction provides a new route for the generation of arylnitrenium ions, a highly selective method for the detection of explosives in mixtures, and offers clues to the carcinogenic activity of nitroaromatics in vivo.