AbstractMetastable peak characteristics, ionization and appearance energy data and isotopic labelling experiments have been applied to a study of the fragmentation behaviour of the molecular ions of the isomeric C4H6O2C acids,cisandtrans‐crotonic acids, methacrylic acid, butenoic acid and cyclopropane carboxylic acid. Prior to the losses of H2O and CH3, all the metastable molecular ions rearrange to [cis‐crotonic acid]+⋅ ions. Loss of H2O, which generates a composite metastable peak, is proposed to yield vinylketene and/or cyclobutenone molecular ions. Detailed mechanisms are presented for the isomerizations of the various molecular ions and for the above fragmentations. Ionized 3‐butenoic and cyclopropane carboxylic acids display a major loss of CO from their metastable ions, a minor process in the other isomers. The metastable peaks consist of two components and these are ascribed to the formation of propen‐1‐ol and allyl alcohol as daughter ions. Some comparative data are presented for the isomeric C5H8O2acids, tiglic acid, angelic acid and sen