Butadiene polymerization in aliphatic and aromatic solvents mediated by a preliminarily formed catalyst consisting of cobalt 2-ethylhexanoate, methylaluminoxane, and diene was studied. It was shown that the catalyst was active at a molar ratio of methylaluminoxane : cobalt: diene = 75-200 : 1 : 20, yielding high-molecular-mass polymers (intrinsic viscosity, 1.8-2.7 dl/g) containing at least 98% cis-1,4-units. Preliminary formation of the catalyst was found to increase its activity in butadiene polymerization at a low Al : Co ratio. In this case, the catalyst retains its high stereospecificity (the amount of cis-1,4-units in the polymer reaches 98% and more) independently of the nature of the solvent used for polymerization.