Polymerization of 1,3-butadiene was conducted with the CoBr2(PPh(3))(2)(Ph: phenyl)-methylaluminoxane (MAO) and MgCl2-supported CoBr2(PPh(3))(2)-Al(CH3)(3) catalyst systems. The microstructure of polybutadiene was analyzed in detail by C-13 NMR spectroscopy after hydrogenation, which indicated that the distribution of the 1,2 and 1,4 units in polybutadiene is more random when the homogeneous CoBr2(PPh(3))(2)-MAO catalyst was used. The microstructure of these polymers was compared with that obtained using BuLi as catalyst.