Thd polymerization of butadiene in benzene in the presence of titanium tetraiodide triisobutyl aluminium catalyst has been studied．It is found that the activity of this catalytic system depends upon the Al／Ti ratio and the temperature of polymerization．For each specified amount of titanium tetraiodide used，there iS a“critical Al／Ti ratio” at which the catalytic system displays highest activity，below which it shows no activity， and above which its activity decreases gradually (Fig．2)．Since the rate of polymerization can be lowered by increasing the AI／Ti ratio without suppression of ultimate conversion(Fig．3)，the polymerization reaction may be controlled with Al／Ti ratio to avoid“heat build—up”in the reaction mixture(Fig．4)．The advantage of this catalytic system is that whatever be the composition of the catalyst(e．g．Al／Ti)and the condition of polymerization (e．g．temperature)， no gel formation OCCURS and the cis-1，4-content of the polymer obtained always exceeds 90％ (Table 2)．The molecular weight(expressed as [n]) of the polymer depends upon two important factors，the amount of titanium tetraiodide used and the temperature of polymerization，with increasing either of which，the molecular weight decreases(Fig．5 and Fig．6)．The effect of AI／Ti on molecular weight is much less significant(Fig．6)． The molecular weight distribution of the polymer obtained with this catalyst is very narrow(Fig．7)．Therefore，it is presumed that the active sites may be of similar type，although the catalytic system itself is heterogeneous in character．