Copolymerization of styrene (St) and butadiene (Bd) with nickel(II) acetylacetonate [Ni(acac)(2)]-methylaluminoxane (MAO) catalyst was investigated. Among the metal acetylacetonates [Mt(acac)(x)] examined, Ni(acac)(2) showed a high activity for the copolymerization of St and Ed giving copolymers having high cis-1,4-microstructure in Bd units in the copolymer. The effect of all alkylaluminum as a cocatalyst on the copolymerization of St and Bd with the Ni(acac)(2)-MAO catalyst was observed, and MAO was found to be the most effective cocatalyst for the copolymerization. The monomer reactivity ratios for the copolymerization of St and Bd with the Ni(acac)(2)-MAO catalyst were determined to be r(St) = 0.07 and r(Bd) = 3.6. Based on the obtained results, it was presumed that the random copolymers with high cis-1,4-microstructure in Bd units could be synthesized with the Ni(acac)(2)-MAO catalyst without formation of each homopolymer. The polymerization mechanism with the Ni(acac)(2)-MAO catalyst was also discussed.