The dimerization of propene was catalyzed by cationic nickel complexes in a two-phase solvent system using organochloroaluminate ionic liquids as the solvent for the catalyst. In ionic liquids containing an excess of strongly coordinating chloride ions, i.e., basic, no activity was observed. In contrast, melts containing an excess of alkylchloroaluminum species, i.e., acidic, stabilized the active cationic nickel species. The reaction products separate as an organic layer. Molecular organochloroaluminum species were extracted, and the composition of the salt was strongly modified. This was circumvented using a salt which contains an excess of aluminum chloride. The propene dimers obtained by this way can be transformed either into ethers or into alkanes to produce high octane number additives for gasoline. The effects of phosphine ligands coordinated on nickel and operating variables were investigated in order to maximize the octane number of the corresponding alkanes and ethers.