In isobutane/2-butene alkylation, chloroaluminate ionic liquid catalysts (CAIL) deactivate fast with time on stream with respect to activity and selectivity. Hence, the effect of anhydrous hydrogen chloride (HCl), which shows co-catalytic behavior, was studied in a batch reactor and its effect on both parameters was investigated. The co-catalyst leads to an increased reaction rate and improves the yield of trimethylpentanes, the primarily desired high-octane compounds of alkylation. Moreover, already deactivated CAIL catalysts are reactivated by gaseous HCl. In addition saturation of the ionic liquid with HCl prior to the reaction effectively suppresses the deactivation of the CAIL catalyst. Thus, the solubility of hydrochloric acid is reported for both the CAIL (IL: [BMIM]Cl/AlCl3, x=0.64) and organic phase to gain a deeper understanding of the reaction system. Finally conjunct polymers dissolved in the ionic liquid were extracted to study their influence on deactivation.