AbstractThe mass spectra of 13C‐labelled thiophenes were studied and the label‐distributions for the C3H3⊕, HCS⊕, C2H2S⊕ and [M CH3]⊕ ions are interpreted in terms of a carbon skeletal rearrangement. From a comparison of the results of 13C‐labelling and D‐labelling, a concurrent hydrogen migration process is demonstrated. The production of HCS⊕ ions is preceded by partial carbon scrambling and hydrogen migration is of minor importance. In contrast, hydrogen scrambling predominates over carbon scrambling in the loss of acetylene.