Halogeno(beta-dithioacetylacetonato)nickel(II) complexes with different alkyl phosphine ligands have been studied in the selective propylene dimerization to 2,3-dimethylbutenes (DMB) in the presence of different organoaluminium compounds. When basic and bulky phosphine ligands, such as tricyclohexyl-(PCy3) and triisopropyl-phosphine ((PPr3)-Pr-1) were used in combination with suitable co-catalysts, such as Et3Al2Cl3, Et2AlCl and EtAlCl2/methylalumoxane (MAO), the best results in terms of productivity (up to similar to 36,000 h(-1)) and yield to DMB (up to 68%) were obtained, A detailed analysis of the composition of C9 cut in some selected experiments allowed to evaluate the octane characteristics of the reaction mixture offering both interesting applicative perspectives and mechanistic information. (C) 2000 Elsevier Science B.V. All rights reserved.