||Isotopic perturbation of equilibrium is used to study the structure of cyclic bromonium and chloronium ions under superacidic conditions. The 13 C NMR spectrum of the five-membered ring generated from ionization of 1,4-dichlorobutane- d 2 exhibits intrinsic isotope shifts only, and is consistent with a symmetric structure. The 13 C NMR spectra of ions generated from 2,3-dibromo-2,3-dimethylbutane- d 3 , 2,3-dibromo-2,3-dimethylbutane- d 6 , and 2,3-dichloro-2,3-dimethylbutane- d 3 (dichloropinacolone?) exhibit small intrinsic isotope shifts and large equilibrium isotope shifts. These species do not exist as symmetric 1,2-bridged structures, but as a rapidly equilibrating mixture of β-halocarbenium ions. Computational investigations suggest formation of a complex containing a solvent molecule and a halonium ion. The mechanisms associated with isomerization of the O -methylethylene oxonium ion and its mono-, di-, tri-, and tetramethyl substituted analogs are also explored. Evaluation of methods and basis sets indicate that the B3LYP/6-31G* level of theory is reliable for geometry optimizations; the higher order MP2/cc-pVTZ level of theory is needed to obtain accurate relative energies. The transition states leading to O -methyl inversion and epoxide opening are characterized. The inversion barrier relative to the symmetric 1,2-bridged oxonium ion is ∼15 kcal/mol for all species; the barrier to ring-opening is 35.3 kcal/mol for O -methylethylene oxonium and decreases with greater substitution of the epoxide carbon involved in cleavage.