Some deuterated hydrocarbons have been prepared and their mass spectra determined. The results indicate that the rearrangements of hydrogen atoms in unsaturated ions are predominantly of the 1–2 type. The loss of ethylene from the parent ion of pent-1-ene does not occur by the mechanism involving a six-membered cyclic transition state as has been suggested. The fragmentations of cyclopentane are shown to occur from the open chain form of the ion rather than from the cyclic form.