The nonamethylcyclopentyl cation shows two types of methyl substituent in the NMR spectrum at low temperatures: four methyl groups undergo rapid circumambulatory migration with a barrier <2 kcal/mol while five methyl groups are fixed to ring carbons The process that equalizes the two sets of methyls has a barrier of 7.0 kcal/mol. C-13 NMR spectra of the (methyl-d(3))-octamethylcyclopentyl cation in SbF5/SO2ClF at -133 degrees C showed four ring carbons (2:2:5:1) indicating that the minimum structure is not symmetrically bridged. Quantum chemical calculations at HF/6-31G(d) and B3LYP/6-31G(d) levels revealed a minimum structure resembling a classical trivalent carbenium ion, but stabilized by hyperconjugation and partial bridging of two quasi-axial beta-methyl groups. This bridging is related to the unique dynamic behavior of the title carbocation.