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The isomerization of the molecular ions of simple ketones1, acids2 and esters3 has been studied intensively. Most of these radical ions studied show extensive hydrogen rearrangements via 3 -, 5-, and 6-membered transition states4. However, the molecular ions of isobutyric acid and its methyl ester5 as well as of some aliphatic ketones6 exhibit an interesting skeletal rearrangement by a 1,2-shift of the C(0H)OH-, C(0H)OR-and C(0H)R-group, respectively. These skeletal rearrangements are of special interest firstly because of the insight gained by these reactions into the reactivity of gaseous organic ions and secondly because of their relevance to the analytical applications of metastable ion techniques. In this connection we have observed the loss of structural identity of certain ketone ions in the CID spectra obviously due to a profound skeletal rearrangement, which cannot be explained by the mechanism elucidated f o r the isomerization of isobutyric acid and related compounds5. Here we report on these rearrangements of ketone ions C7H140' ' which occur by a back-reaction in intermediate ion/molecule complexes. The structures of the molecular ions of compounds 1 -8 ZAB-2F mass spectrometer7 (Table 1).were investigated by their C I D spectra obtained with a VG Structures of isomeric C,H14O+' ions 1 -8