The selective dimerization of styrene to 1,3-diphenyl-1-butene over Pd(beta-diketonate)(2)/BF3OEt2 catalyst systems in both "phosphine-free" and "phosphine-modified" fashions has been investigated. For the Pd(acac)(2) +2PR(3) + 7BF(3)OEt(2) catalyst system the turnover of 75,000 mol styrene/mol palladium for 7 h was reached at 70 degrees C with selectivity to dimers of 93%. Styrene dimers up to 95% consists of trans-1.3-diphenyl-l-butene. Catalytic activity and selectivity can be controlled by varying the reaction parameters, The nature of substituents on the beta-diketone did not affect the conversion of styrene. However, the selectivity to dimers increases almost linearly with decreasing acidity of beta-diketonate ligand. The nature of the phosphine ligand has a strong effect on the conversion of styrene, but not on the selectivity. The order of the decrease in conversion parallels the increasing basicity (electronic effect) rather than steric properties of the phosphines. Reaction products were characterized with H-1 and C-13 NMR, IR. and GC-MS spectroscopies and GC analysis, Palladium hydride complexes are likely to be catalytically active species. (c) 2005 Elsevier B.V. All rights reserved.