The effect of different promoters on activity and selectivity of Lewis-acidic chloroaluminate ionic liquid catalysts was studied for isobutane/2-butene alkylation. When tert-butyl halides are used as promoters, the active species of the alkylation reaction, which is the tert-butyl cation, is directly generated whereas upon catalysis with Bronsted-acid supported ionic liquids, this species is indirectly provided through a hydride shift between protonated 2-butene and isobutane. Experimental results both from batch and continuously operated liquid phase alkylation reactors indicate, that tert-butyl halides are able to speed up the reaction rate significantly and shift the C-8-selectivity towards the desired high-octane trimethylpentanes (TMPs). However, secondary reactions like oligomerization and cracking could not be suppressed by the use of this additives and high deactivation rates in continuous opperation were observed. Suggestions are made, how the product composition is effected by the additive and how the promoted IL-catalyst system is deactivated with time on stream.