Abstract In order to examine a possibility of keto-enol tautomerism in molecular and fragment ions, diethyl di-n-butylmalonate (Ia), methyl 2-n-butylhexanoate (IIa), and 2,2-di-n-propyl-5,5-dimethyl-1,3-cyclohexanedione (IIIa) were synthesized and their mass spectra were compared with those of respective mono-alkyl derivatives, Ib, IIb, and IIIb. The first step of the fragmentation of each dialkyl compounds is the cleavage of one of the alkyl side chains by McLafferty rearrangement to give an enol fragment ion (E). Comparisons of the behavior of this enol fragment ion with that of the corresponding molecular ion (K) of mono-alkyl derivatives indicate that the keto and enol forms of these compounds exhibited entirely different fragmentation patterns. Thus, the enol fragment ion of m/e 216 from Ia loses C3H7· through the allylic cleavage of an enol double bond to give an abundant m/e 173 peak, while the keto ion of m/e 216 from Ib loses C4H8 to afford a strong m/e 160 peak. In a similar manner, the enol fragment ion of m/e 130 from IIa gives an intense peak at m/e 87, and the keto ion of m/e 130 from IIb yields an ion at m/e 74. In the spectrum of IIIa, a peak at m/e 140 that is observed in the IIIb spectrum did not appear. These results clearly suggest that no keto-enol tautomerism takes place in these ions in a mass spectrometer.