Syndiotactic polystyrene (sPS) was synthesised using homogeneous catalytic systems based on titanium derivatives (tetraethyl orthotitanate, (cyclopentadienyl)titanium trichloride), methylaluminoxane (MAO) and aluminium alkyls (trimethylaluminium, triisobutylaluminium). The polymers were characterised by gel permeation chromatography (GPC) and nuclear magnetic resonance (n.m.r.) spectroscopy.Trimethylaluminium was found to decrease both the catalytic activity and the polymer molecular weight. Triisobutylaluminium causes a reduction in activity only for low ( < 600) methylaluminoxane/titanium molar ratios. While the molecular weight decrease is almost linear with the increase of the amount of aluminium in the catalytic system in the case of trimethylaluminium, a very sharp decrease of molecular weight is observed by addition of triisobutylaluminium even in moderate amounts. The results were interpreted on the basis of the different ability of the two aluminium alkyls to behave as chain transfer agents. Moreover, the reactions of termination and re-initiation of the polymer chains were examined taking into account the end-group structures,and a polymerisation mechanism to explain the experimental results was put forward.