Propylene dimerization has been studied in a biphasic system formed by [Ni(MeCN)(6)][BF4](2) dissolved in 1-butyl-3-methylimidazolium organochloroaluminate ionic liquid (IL). The activity and dimerization selectivity show a high dependence on the reactional parameters and evidence of interfacial phenomena feature has been obtained. The isomerization of C-6 olefins could be minimized to 13% using a control of dimerization temperature and propylene pressure. (c) 2007 Elsevier B.V. All rights reserved.