AbstractMass spectra of the three isomeric vinylimidazoles have been compared and the structures of the fragment ions [C3H4N2]+·and [C5H5N2]+have been investigated by collisionally activated dissociation mass spectrometry. The greater part of the non‐decomposing ionsm/z68 from 2‐vinylimidazole and from 2‐imidazolecarboxylic acid methyl ester, and a minor part of this ion formed from the free acid, all have the same structure: the imidazole ring system, with hydrogens at both nitrogen atoms but none at C(2). An analogous structure, with an ethyl group at C(2), is proposed for them/z93 ion from 2‐vinyli