Nickel(II)-diiminophosphorane complexes combined with alkylaluminum co-catalysts are active for the oligomerization of ethylene under biphasic reaction conditions. As observed in homogeneous phase, the selectivity (dimers vs higher oligomers) is related to the nature of the diiminophosphorane ligands. In 1-n-butyl-3-methylimidazolium organoaluminate ionic liquids, an enhancement of the catalytic activity and convergence of the selectivity are observed upon repeating cycles. These data suggest that organoaluminate anions replace the diiminophosphorane ligands in the coordination sphere of the active nickel species. (C) 2003 Elsevier Inc. All rights reserved.