1,3-Butadiene polymerization was conducted at 0degreesC with cobalt dichloride (CoCl2) activated by different cocatalysts: MAO (methylaluminoxane), dried-MAO (substantially free of trimethylaluminum), MMAO (triisobutylaluminum-modified methylaluminoxane), dried-MMAO (substantially free of trialkylaluminum) and TMA (trimethylaluminum). MAO and MMAO were effective cocatalysts for 1,3-butadiene polymerization. CoCl2 activated with MAO showed slowly-initiated living polymerization which has been reported previously, whereas, that with MMAO showed deactivation. Neither dried-MAO nor dried-MMAO, TMA activated the catalyst, whereas dried-MAO-TMA (mixtures of dried-MAO and TMA) and dried-MMAO-TMA (mixtures of dried-MMAO and TMA) allowed the polymerization. These results indicated that trialkylaluminums were necessary with MAOs for generating the active species. Dried-MAO-TMA caused frequent chain transfer reactions, whereas dried-MMAO-TMA showed the living like polymerization not only with slow initiation but with slow propagation also, suggesting that the nature of the active species was strongly dependent on the structure of alkylaluminums and MAOs. The microstructure of the resulting polymers analyzed by H-1 and C-13 NMR was cis-1,4 (98-97%); the percentage slightly decreased in the following order: MAO > dried-MAO > MMAO > dried-MMAO, accompanied by the increase of trans-1,4 and 1,2-inserted units.