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Electron ionization-induced loss of SO 2 from 2-nitrodiphenyl sulfide leads to the same ionic structure, or mixture of structures, as loss of N 2 from the molecular ion of N 1 -phenylbenzotriazole. Ab initio calculations are in favor of the [C 6 H 4 -N-C 6 H 5 ] + distonic nitrenium radical cation instead of the N-phenylhenzo-Δ 2 -azirinium structure for the product ion. Collisionally activated dissociation mass-analyzed ion kinetic energy (CAD-MIKE) spectra of the four chloro isomers [3- or 4-Cl-C 6 H 3 -N-C 6 H 5 ] + and [C 6 H 4 -N-C 6 H 4 -3 or 4-Cl] + show that these ions are non-interconverting structures and eliminate regioselectively their nitrene moiety. Comparison with the CAD-MIKE spectra of the product ions of the elimination of N 2 from N 1 -3(or 4)-chlorophenylbenzotriazoles demonstrates that the loss of SO 2 from 2-nitrophenyl aryl sulfides occurs in the gas phase via an ipso rearrangement involving a [1,4] migration of the aryl group.