Previous experimental studies of the water elimination from ionized ethyl acetate (EA-1) and its enol, CH 2 =C(OH)OC 2 H 5 ·+ (EA-2), provide no mechanistic proposals, but note that the reaction is complex because all hydrogen atoms and both oxygen atoms participate. Loss of H 2 O is the predominant process observed for the metastable ions, yielding ionized methyl vinyl ketone, CH 3 C(=O)CH=CH 2 ·+ (MVK), as the product ion. Metastable keto-alcohol ions CH 3 C(=O)CH 2 CH 2 OH ·+ (HB-1) also abundantly lose H 2 O, yielding MVK at the thermochemical threshold. Using the CBS-QB3 model chemistry and complementary RRKM calculations, we have examined plausible mechanisms for the water elimination from metastable ions EA-1/2 and HB-1. Our calculations support the proposal that (i) ions HB-1 dissociate into MVK at the thermochemical threshold via a 1,4-H shift and a consecutive [1,2]-hydroxycarbene shift, and (ii) ions EA-1/2 undergo a three-step isomerization reaction into HB-1, which serves as the precursor for the ensuing water elimination. It is further shown that synergy between theory and experiment leads to a complex, yet transparent, mechanistic picture of the isomerization and dissociation behaviour of ionized ethyl acetate, which accounts for the labelling results.Key words: tandem mass spectrometry, CBS-QB3 model chemistry, reaction mechanism, hydroxycarbene shift.