The potential energy surface for the [CH5N]+˙ system has been investigated using ab initio molecular orbital calculations with large, polarization basis sets and incorporating valence‐electron correlation. Two [CH5N]+˙ isomers can be distinguished: the well known methylamine radical cation, [CH3NH2]+˙, and the less familiar methylenammonium radical cation, [CH2NH3]+˙. The latter is calculated to lie 8 kJ mol−1 lower in energy. A substantial barrier (176 kJ mol−1) is predicted for rearrangement of [CH2NH3]+˙ to [CH3NH2]+˙. In addition, a large barrier (202 kJ mol−1) is found for loss of a hydrogen radical from [CH2NH3]+˙ via direct N—H bond cleavage to give the aminomethyl cation [CH2NH2]+. These results are consistent with the existence of the methylenammonium ion [CH2NH3]+˙ as a stable observable species. The barrier to loss of a hydrogen radical from [CH3NH2]+˙ is calculated to be 140 kJ mol−1.