Investigation on the chain transfer in the polymerization of butadiene with MoCl4OC6 H17-(i-Bu)2AlO as catalyst has been carried out. In this catalyst system it was foundthat the monomer and MoCl4OC8H17 played a main part in the chain transfer. The rate of transfer is first order with respect to the concentration of both monomer and MoCl4OC8H17 and 1/2 order with respect to the concentration of the aluminium alkyl.