Dimerization of propylene was studied with several bis(salicylaldehyde)nickel(II) complexes in combination with aluminum co-catalysts, eventually in the presence of ancillary phosphine ligands. The effects of the type and relative amount of aluminum co-catalyst as well as the nature of nickel precursors were examined. The results clearly indicated that the nickel precursors characterized by the presence of either bulky substituents or electron-withdrawing groups on the salicylaldehyde ligands displayed high activity. The catalytic performance was also remarkably modified in the presence of phosphine ligand, and the regioselectivity in the dimers was directed to 2,3DMB with incorporation of basic and bulky PCy3 into the nickel catalysts, especially at the temperature below 0 degreesC. A preliminary UV-Vis study was performed in order to detect the variation of nickel species involved in the dimerization process. The bis(2-hydroxy-1-naphthaldehyde)nickel (1d) system exhibited an extremely high productivity (TOF up to 262,000 h(-1)) as well as a moderate regioselectivity to 2,3DMB (65.6%) in the dimerization of propylene under appropriate reaction parameters. (C) 2003 Elsevier Science B.V. All rights reserved.