The effect of the composition of the 1-butyl-3-methyl-imidazolium organochloroaluminate ionic liquid on the activity and distribution of the products of I-butene dimerization catalyzed by a nickel complex under biphasic conditions is described. Three classes of ionic liquids have been employed, the first one with an aluminum (AlCl3) molar fraction of 0.50, the second one with an aluminum molar fraction of 0.55 and the third one with an aluminum molar fraction of 0.65. These ionic liquids were modified with different amounts of AlEtCl2 co-catalyst. An increase in the amount of co-catalyst added to the ionic liquid increases the global activity of the system, except for an AlCl3 molar fraction of 0.50. Changing the cation from 1-butyl-3-methylimidazolium to 1-butyl-2,3-dimethylimidazolium gives rise to no modification in selectivity towards dimer products. (c) 2006 Elsevier B.V. All rights reserved.