Acidic ionic liquids (IL) are attractive alternative catalysts in refinery alkylation of i-paraffins with light olefins, but the intrinsic kinetics and influence of mass transfer on the effective kinetics in this biphasic system is still unknown. Solubility measurements were conducted (largely with neutral, nonacidic ILs) using mixtures of i-hexane/2-hexene and i-pentane/2-pentene, respectively, to determine the Nernst partition coefficient and thus the maximum concentration of olefins and paraffins in the IL. Thereafter, kinetic studies were carried out both in a stirred and nonstirred batch reactor using i-hexane/i-hexene and i-pentane/1-pentene. In the static system, the concentration profile of the particular olefin in the organic phase was measured. The experimental results are in good agreement with the simulation based on the interplay of the chemical reaction in the ionic liquid as well as of the external and internal mass transfer to the interface and into the IL. The effective reaction rate of alkylation is proportional to the interfacial surface area between organic and IL phase: The intrinsic chemical reaction is very fast which leads to a strong mass transfer limitation of the olefin into the IL phase. Hence, the effectiveness (compared to utilization of the entire IL phase) is very low, and the alkylation reaction takes place in a very thin layer with a thickness of only around 5 mu m.