The cationic nickel(II) complexes [(eta(3)-C3H5)Ni(SbPh3)(2)][B(3,5-(CF3)(2)C6H3)(4)] (2) and [(eta(3)-C3H5)Ni(SbPh3)(2)][Al(OC(CF3)(3))(4)] (3) were prepared and their polymerization activities in aqueous systems were compared with [(eta(3)-C3H5)Ni(eta(6)-BHT)][B(3,5-(CF3)(2)C6H3)(4)] (1) (BHT = 3,5-di-tert-butyl-4-hydroxytoluene). Cispolybutadiene is formed in nonaqueous solution polymerization. The presence of water decreases activity and polymer molecular weight and increases trans incorporation of butadiene units. In emulsion polymerization, stable polymer dispersions can be obtained with complex 2 and 3, respectively. The microstructure is strongly altered by comparison to nonaqueous polymerization. Trans-polybutadiene is formed with activities of up to 1.5 x 10(3) mol (butadiene) mol (Ni)(-1) h(-1). The semicrystalline latex particles have sizes of ca. 200 nm, with a monoclinic structure.