The effects of residual trimethylaluminum (TMA) in methylaluminoxane (MAO) and external alkylaluminum (AlR3) on styrene syndiotactic polymerization with CpTi(OBz)(3) as a catalyst precursor have been investigated by comparison of the polymerizations using a series of MAOs containing various amounts of residual TMA. The results indicated that the residual TMA. plays a deciding role in the reduction of Ti and promotes formation of the active centers for styrene polymerization. The variations in the catalytic activity and molecular weight of the polymer caused by additions of external AlR3, AlMe3, AlEt3, Al(i-Bu)(3), and ALEt(2)Cl, into the catalyst systems are quite ;different, depending the properties of MAO used and the type of the external AlR3. It was found that there is an optimum range of concentration ratio of the free AIR,, including the residual TMA and external AlR3, to the total Al compounds, 25-35 mol %, for maximum catalytic activities. The catalytic activities decrease at the ratios either above or below the range.