The stereoregular emulsion polymerization of butadiene was performed with a cobalt catalyst (J. N. Henderson et al., U.S, Patent 4,429,085, 1984; H. Ono et al., Jpn. Open Patent 85-147410, 1985; H. Ono et al., Jpn. Open Patent 85-161408, 1985; H. Ono et al., Jpn, Open Patent 85-14711, 1985; H. Ono et al., Jpn. Open Patent 86-69818, 1986; H. Ono et al., Jpn. Open Patent 86-23610, 1986). This is the first report of a successful highly stereoregular emulsion polymerization with water as the solvent (H. Ono et al., Jpn. Open Patent 85-14711, 1985; H. Ono et al., Jpn. Open Patent 86-69818, 1986; H, Ono et al., Jpn. Open Patent 86-23610, 1986); water is usually recognized as a strong inhibitor of transition-metal polymerization. By analyzing the addition order effect of the ingredients, we show that the reduction of cobalt in the presence of butadiene is necessary for the polymerization. Radical-polymerization inhibitors such as phenols did not terminate the polymerization reaction, but sulfur did. Moreover, the valence of the reduced cobalt butadiene complex catalyst is considered zero. The zero-valent cobalt is classified as a soft acceptor, butadiene is a soft donor, and water is a hard acceptor. In this article, we discuss the reason butadiene could be polymerized stereoregularly by an emulsion system.