The selectivity-determining step in Heck arylation of styrene, the migratory insertion, has been modeled by DFT. In particular, the regio-selectivity has been studied as a function of the trans effect of several sets of small model ligands with different sigma-donor and pi-accepting abilities. It has been shown, both by analyzing the geometries and comparing the activation energies, that the TS for branched product formation is more sensitive than the TS leading to linear product, although most ligand combinations give a preference for the linear product. The branched TS is strongly destabilized by any strong ligand in a position trans to the alkene in the pre-insertion intermediate. For the ligand trans to the migrating group, a strong trans effect accelerates the reaction, more for the branched than for the linear product formation. On electronic grounds, the regio-selectivity and rate can be adjusted by considering not only the trans effect of a ligand, but also by controlling the position they will have in the migratory insertion step. Very importantly, the trans effect change in the selectivity-determining TS will be expected to give rise to Halpern-type selectivity, with the consequence that product distributions cannot be reliably predicted from observable intermediates. (C) 2010 Elsevier B.V. All rights reserved.