The isomerization of aliphatic amine radical cations via intermediate [cyclopropane-NH3](+center dot) and [cyclopropane-amine](+center dot) ion-neutral complexes was studied experimentally with double-focusing mass spectrometers and computationally with composite ab initio methods. The results examine and extend Audier's suggestion that primary amine radical cations with alkyl substituents at the beta- and/or gamma-carbon atoms isomerize via transient complexes of NH3 and alkyl cyclopropanes; these are formed by ring closure of the easily accessible gamma-distonic isomers. Ionized amines with substituents at the alpha-carbon may react analogously when trialkyl cyclopropane complexes can be formed. Isomerization via complex intermediates is a major reaction pathway when the internal energy of the amine radical cation is less than that required for simple CC-bond cleavage. Complexes of unsubstituted or monosubstituted ionized cyclopropanes rarely contribute to the isomerization reactions. Secondary and tertiary amine radical cations do not undergo isomerization via cyclopropane intermediates, whereas aliphatic ether radical cations do.