The stereospecific polymerization of butadiene catalysed by homogeneous systems formed by combination of trialkyl aluminium with β-diketone chelates of rare earth ele-ments has been studied．It has been shown that combinations of the following rare earth chelates with triethyl aluminium could induce cis-1，4 stereospecific polymerization of butadiene： NdB3(B=benzoylacetonate)，NdB，NdBTA(BTA=benzoyltrifluoroacetonate)，NdTTA(TTA=thenoyltrifIuoroacetonate)，PrB3，PrB，PrBTA，PrTTA，YB3，YBTA，and LaBTA．These homogeneous systems appear to possess much higher catalytic activity than those of the heterogeneous systems formed by combination of triethyl aluminium with halides of lanthanons studied earlier．Up to a certain extent of catalyst concentration，the catalytic activities of these systems increase with increasing ratio of Al/chelate or chelate/butadiene (molar ratio)．The polymerization has an induction period and it is found that the rate of polymerization is of first order with respect to the monomer concentration and approximately of first order to the chelate concentration．With the NdB3-Al(C2H5)3 system．the activation energy of polymerization has been determined tO be 13.0±0.5 kcal/mole．Both polymerization rate and microstructure of polybutadiene depend upon the nature of the rare earth metal in the chelates rather than that of the ligands in various chelates，and the praseodymium and neodymium chelate systems gave higher catalytic activity．It is interesting tO note that polybutadienes obtained with these homo-geneous systems always have a 1，2-structure content less than 1％，with 1ittle or even no geI formation．The intrinsic viscosities of the polymers obtained varying from 4一5 to 1，depend on the Al/chelate and chelate/butadiene ratios．Variation in alkyl groups on the aluminium component results in a marked difference in catalytic activity．The Al(i-C4H9)3 system exhibits low activity and Al(C2H5)2Cl system is inactive．