The effects of phosphine ligands on the polymerization of 1,3-butadiene with Co(2-ethyl hexanoate)(2)/AlEt2Cl/H2O were investigated, Although the polarizability of phosphine ligands, i.e. the ratio of the average dipole moment to the molecular volume, had moderate impact on 1,4-cis stereoregularity, a good correlation was observed between polarizability and 1,4-cis stereoregularity regardless of the electronic character (pi-acceptor or sigma-donor) of phosphine ligands. On the other hand, neither catalytic activity nor the molecular weight (M-w) of polybutadiene showed a good correlation with polarizability. Catalytic activity was affected only when phosphine ligands were inordinately strong pi-acceptors and sigma-donors. High conversions of about 90% were obtained when phosphine ligands were moderate pi-acceptors and sigma-donors, regardless of their steric bulk. On the other hand, the electronic character of phosphine ligands significantly influenced the M-w of polybutadiene. (C) 2003 Elsevier B.V. All rights reserved.