The oligomerization of n-butenes in a two-phase catalyst system consisting of a Lewis acidic ionic liquid (IL) and a nonpolar phase is described. A highly active catalyst system with NiCl2(PMe3)(2) was investigated, resulting in 91% dimers with N-methylpyrrole as a buffer. The reaction was found to be dependent on the molar ratio of the IL to the catalyst. Additionally, the influence of the Lewis acid was investigated and AlCl3 was determined as the most suitable for activating the nickel complex. The concentration of AlCl3 played an important role in achieving high catalyst activity and dimer selectivity. Several reactions with different pyrrole derivatives demonstrated that the use of a buffer with low steric hindrance was also important with regard to increased dimer selectivity.