The deuterium distribution observed in isobutane recovered after short contact times with the DF-SbF5 superacid at 0 degrees C shows that a very fast reversible protonation of all C-H bonds occurs before ionization of the alkane, in accord with the Olah sigma-basicity concept. Comparison of the amounts of hydrogen with the amount of tert-butyl ions generated during ionization shows that the reaction is purely protolytic in HF containing up to 20 mol % SbF5, but becomes oxidative at higher concentrations.