Kinetics of polymerization of butadiene with soluble Ziegler-type catalyst CoCl2·4C5H5N一(C2H5)2AlCl have been studied by means of dilatometer．The results indicate that the polymerization rate in benzene is first order with respect tO both mono-met and cobalt chloride concentration．and is one-half order witll respect to diethyl alu-minium chloride concentration．The activation energy of polymerization has been deter-mined to be 8.7±0.5 kcal／mole．When the solvent benzene is partly substituted by heptane，the polymerization rate is decreased and can be represented as：-dm-dt={β[Co][Al]1／2—δV}[M] where V represents the volume percentage of heptane in solvent，β and δ are constants．If small amount of certain electron donors is added to the polymerization system，the rate equation may be represented as： -dm-dt={β[Co][Al]1／2—a[D]}[M] where [D] is the concentration of electron donor and a represents the characteristic cons-tant of the electron donor．The value of a for different electron donors decreases in the following order：triethylamine>tetrahydrofuran>ethyl ether>anisole．