Abstract[C3H3N2]+ions have been generated under electron impact conditions from some monosubstituted pyrazoles C3H3N2R. Collision‐induced dissociation (CID) mass spectra of deuterium‐labelled precursors suggest that the majority of the [C3H3N2]+ions formed from 1‐nitro‐ and 4‐bromo‐pyrazole retain their cyclic structure, whereas the ions from 3(5)‐bromopyrazole are mainly linear. This is confirmed by the relative values observed for the overall cross‐sections for CID and for ion loss. An isotope effect of the order of 1.5–1.9 has been found for the collision‐induced loss of H˙from [C3H3N2]+, generated from 3(5)