n-Butenes are selectively dimerized by cationic nickel complexes such as [Ni(MeCN)(6)][BF4](2), [Ni(MeCN)(6)][AlCl4](2), [Ni(MeCN)(6)][ZnCl4] immobilized in 1-n-butyl-3-methyl-imidazolium chloroethylaluminate ionic Liquid in typical two-phase catalytic reactions. Turnover frequencies up to 2.2 s(-1) were achieved under very mild reaction conditions (1.08 bar and 281 K). The activity depends on the nature of the catalyst precursor. Under these conditions, between 93% and 96% of octenes were produced with a branching index of 1.2-1.3. The selectivity of the system indicates the existence of a cationic nickel-hydride complex as the catalytic active species. The products are easily separated from the reaction mixture by simple decanting and the recovered ionic catalyst solution can be reused several times without significant loss in the catalyst performance.