The influence of several phosphorus(III) ligands upon the activity and the stereoregulation ability of the soluble organometallic complex [Ni(eta(3)-(2-methylallyl))(eta(4)-COD)]PF6 (COD = 1,5-cyclooctadiene), 1, is studied. There is a general increase of the catalytic activity which can be associated with the P(III) ligand's lability. A detailed analysis of the mass distribution and microstructure is carried out by GPC and H-1 and C-13 NMR of the solvent-fractionated polymers. In the majority of cases, regioregular low molecular weight polymers with the structure PhCH=CH2[CH(Ph)CH2]nCH(Ph)CH3 are obtained. The addition of bulky phosphines, such as P(o-Tol)(3) (o-Tol = o-tolyl) and PCy(3) (Cy = cyclohexyl), affords highly isotactic oligomers (approximate to 90%). For the latter, the regioselectivity of the chain growth is broken during the insertion of the last monomer, which results in the formation of a tail-to-tail end group, CH2=C-(C6H5)CH(C6H5)CH2-. The superimposed effects of eta 3-coordination o