The electron impact (EI) mass spectra of allyl aryl selenides showed abundant molecular ions and many fragment ions containing the selenium atom. a-Cleavage is the dominant process in the fragmentation of selenides, and cleavage product ions are characteristic of the substituents. In the case of 3-methyl allyl and related aryl selenides, characteristic d -hydrogen migration to the selenium atom is observed. A McLaffertytype rearrangement is found in benzyl allyl selenides and substituted alkyl allyl selenides. The charge on the rearrangement products preferably remains on the fragments containing the phenyl group. The [M ร SeH] , [M ร CH 3 ] and [M ร C 2 H 4 ] ions are found only in the EI mass spectrum of allyl phenyl selenide, and are attributed to a Claisen rearrangement in the source of the mass spectrometer. All structurally informative fragmentation processes are supported by collision induced dissociation spectra of molecular ions. The fragmentation patterns found in methane chemical ionization (CI) spectra of the selenides were significantly different from those observed in EI. The EI and CI mass spectra of analogous sulfides showed similar behaviour to that observed in the corresponding selenides.