Solvolysis products of 3-azetidinyl chlorides, tosylates, and mesylates have been interpreted previously to indicate that these reactions proceed by azabicyclo[1.1.0]butyl cationic intermediates. Whether these cations are formed by direct ionization to 3-azetidinyl cations followed by collapse to the bicyclic ion or are formed with anchimeric assistance by the lone pair of electrons on nitrogen is unclear. This investigation was initiated to assess the relative stability of these bicyclic cations and their isomeric 3-azetidinyl and aziridinylmethyl cations. All ab initio methods investigated suggest that the bicyclic ions (1) are much more stable than the corresponding 3-azetidinyl cations (3) and that transition states for conversion of the bicyclic ions to azetidinyl carbocations are not acheivable from the bicyclic ions. Hartree-Fock ab initio calculations on N-methyl (and N,2-dimethyl) bicyclic ions and their isomeric aziridinylmethyl cations (2) indicate that the bicyclic ions are significantly more stable than are the isomeric partially ring-opened cations, and that transition states (4) for conversion of the bicyclic ions to the corresponding aziridinylmethyl carbocations are probably energetically unattainable. Hartree-Fock theory predicts that the Nmethyl-2-phenylbicyclic ions are slightly less stable than the resulting aziridinylmethyl cations. Calculations which include electron correlation (MP2) indicate, however, that all bicyclic ions investigated are more stable than any of their isomeric carbocations.