The homopolymerization of butadiene in the presence of a rare earth-containing catalyst system was examined at 60 degrees C. The catalyst system consists of neodymium octanoate, diethylaluminium sesquichloride and triisobutyl aluminium in n-heptane. This catalyst shows a high activity at 120 min polymerization time and yields polymers with a cis-1,4-content of > 96%. The catalyst concentration was varied and the behaviour of molar masses and molar mass distribution, conversion and microstructure of polymers is illustrated. Attention is focussed on molar mass distribution of cis-1,4-polybutadiene samples which can be unimodal or bimodal in dependence on the preparation conditions.