Ionic heats of formation and collisional activation (CA) mass spectra for fifteen isomeric or tautomeric [C4H4O]+• ions are described. Twelve of these species can be characterised by means of their CA mass spectra alone. The relative thermodynamic stabilities of keto–enol tautomers and the ease of interconversion of some ions are discussed. It is emphasised that CA spectra alone (i.e. in the absence of other information, such as heat of formation, etc.) may not easily be correlated with the appropriate ion structures.