Hydrogen/deuterium exchange was observed by H-2-NMR spectroscopy at the CH3 groups of the long-lived alkanoyl cations CH3CH2CO+ (4), (CH3)(2)CHCO+ (8), and (CH3)(2)CCO+ (13) when treated with excess DF .-SbF5 superacid. The intermediacy of the corresponding protio(deuterio)acylium dications is suggested to account for the exchanges. Under similar conditions, no exchange was observed in the acetyl ion CH3CO+ (1) in DF . SbF5, but at the same time its electrophilic reactivity is greatly enhanced in superacids. The acetyl cation in a superacidic medium also abstracts tertiary hydrogens of isoalkanes to give protonated acetaldehyde. Density functional theory calculations at the B3LYP/6-31G** level were carried out to support the suggested exchange mechanism as well as lack of exchange in the acetyl cation. On the basis of the calculated results, two alternative mechanisms are also suggested for the Nenitzescu rearrangement of pivaloyl chloride in isobutane with excess AlCl3.