In mass spectrometry, the primary fragmentation mechanism is homolytic cleavage (homolysis), where a bond breaks such that each fragment retains one electron from the bond, forming radical cations. This is because the ionisation process (electron bombardment in EI-MS) produces radical cations. Heterolytic cleavage (both electrons to one fragment) can also occur in some rearrangements, but homolytic cleavage is the predominant mode.