AbstractAb initiomolecular orbital theory has been used to study the ionization of methane, ethane, propane,n‐butane, isobutane, cyclopropane and cyclobutane. For methane radical cation, the preferred structure hasC20symmetry and a pair of long C—H bonds, resembling a complex of CH+2with H2. This appears to be the only significant minimum on the CH+4surface. For the remaining acyclic systems, the calculated structures of lowest energy (e.g., CH3‐CH2—CH+3, CH3‐CH2—CH2‐CH+3) are characterized by the extreme lengthening of a single C—C bond. However, alternative structures with two or more elongated C—C bonds have very low relative energies. The main structural conclusion for the alkane and cycloalkane radical cations is that the potential energy surface connecting various possible isomeric structures is generally very flat. Facile wide amplitude distortions and scrambling processes are therefore expected in these ions. The large structural changes which accompany ionization are manifested in large differences between vertical and adiabatic ioni