The PR3-CS2 adduct displays an unusual promoting effect on the activity of butene oligomerization catalyzed by nickel complexes in biphasic dialkylimidazolium organochloroaluminate ionic liquid (IL) systems. The promoting effect on the activity was studied and was found to be dependent on the nature of the anionic aluminum species present on the IL, making it observable only in acid ILs. (c) 2006 Elsevier Inc. All rights reserved.