Previous experimental studies of the water elimination from ionized ethyl acetate (EA-1) and its enol, CH2=C(OH)OC2H5·+ (EA-2), provide no mechanistic proposals, but note that the reaction is complex because all hydrogen atoms and both oxygen atoms participate. Loss of H2O is the predominant process observed for the metastable ions, yielding ionized methyl vinyl ketone, CH3C(=O)CH=CH2·+ (MVK), as the product ion. Metastable keto-alcohol ions CH3C(=O)CH2CH2OH·+ (HB-1) also abundantly lose H2O, 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.