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The charge-reversal (CR) mass spectra of the enolate ions of heptanal and ten isomeric heptanones, of cyclohexanone, of cycloheptanone, of isomeric methylcyclohexanones, of isomeric ethylcyclohexanones and of the iso meric monoterpene ketones camphor, fenchone, pulegone and thujone were obtained by deprotonation using OH under chemical ionization conditions followed by collision of the [M - H] ions with helium in the second field-free region of a VG ZAB 2F mass spectrometer. The CR mass spectra were evaluated by similarity index (SI) values. Characteristic of the CR mass spectra of the open-chain enolates are fragment ions formed by [alpha]-cleavage. However, the CR mass spectra are dominated by peaks of small hydrocarbon ions, particularly in the case of cyclic and bicyclic enolates. The CR mass spectra of enolates of linear heptanones differing in the position of the carbonyl group can be easily correlated with the structure of the parent ketone. The CR mass spectra of enolates of isomeric heptan-2-ones differing only in the degree of branching of the alkyl group are similar, but can be distinguished by the SI values. The CR mass spectra of the enolates of the isomeric cyclic and bicyclic ketones studied are more or less identical and cannot be used for structural assignment.