No / Several series of styrylbenzoxazoles of general formula XC6H3(NCO)CH=CHC6H4Y [X= F, Cl or Br; Y = H, F, Cl, Br, CH3 or CH3O] have been investigated by positive ion electrospray and electron ionization mass spectrometry. These compounds, many of which are biologically active or have pharmaceutical potential, show in their electrospray spectra strong peaks for MH+ ions, which undergo relatively little fragmentation. The electron ionization spectra are extremely clean, being dominated by the loss of an atom or radical, Y*, from the ortho position of the pendant ring, by a rearrangement that may be interpreted as a proximity effect. The resultant [M-Y](+) ions are exceptionally stable and rarely undergo further fragmentation. The analytical value of this proximity effect, which is analogous to intramolecular aromatic substitution, in revealing the presence of a substituent in the pendant ring and determining its position, is emphasized. Elimination of a species (including H* or F*) derived from an ortho substituent in the pendant ring occurs even when apparently more favourable alternative fragmentation is possible by direct cleavage of the C-X bond (X = Cl or Br) in the benzoxazole ring.