In contrast to the numerous studies of the mass spectrometric fragmentation behavior of saturated aliphatic ketones, that of unsaturated ketones has received only scant attention. The present article is concerned with high resolution measurements and deuterium labeling studies of 17 a&unsaturated ketones in order to define their fragmentation modes after electron-impact. Several hitherto undescribed cleavage processes were thus uncovered. For instance a,&unsaturated methyl ketones (type A) eliminate acetone from their molecular ions through a double hydrogen transfer process. Such a reaction is suppressed in the presence of an a-alkyl group (type B) where it is superseded by the loss of this alkyl substituent through an apriori unfavorable rupture of a vinylic bond. In those a$-unsaturated ketones (type C) which can undergo the McLafferty rearrangement, evidence is presented for the occurrence of subsequent methyl loss-a process which seems to be associated with the presence of the conjugated double bond. Whether this is due to prior ketonization of the enol, contrary to the behavior of saturated alicyclic ketones, remains to be determined. * For part CXCVII see ref. 1. 18 273 * Similar exchanges of hydrogen and deuterium involving six-membered transition states have been encountered previously.12 * This deuterated reagent was prepared from bromoacetone-d, and triphenylphosphine. The bromoacetone-d, was prepared by brominationZ6 of acetone-d,.