The polymerization of styrene was studied in benzene solution with two catalyst systems: TiCl 3 –Al(C 2 H 5 ) 3 and VCl 3 –Al(C 2 H 5 ) 3 . The reaction was carried out at atmospheric pressure, temperatures of 40–70°C., [monomer] = 0.695–5.75 moles/l., [TiCl 3 ] = 0.12–1.2 g./l., and [alkyl] = 0.001–0.016 mole/l. For polymerizations with TiCl 3 –Al(C 2 H 5 ) 3 catalyst, the rate of polymerization was found to be first-order with respect to TiCl 3 , proportional to [monomer] 1.5 and for [Al(C 2 H 5 ) 3 ] > 0.002 mole/l., proportional to [Al(C 2 H 5 ) 3 ] −1/2 . For [Al(C 2 H 5 ) 3 ] < 0.002 mole/l., the rate passed through a maximum and decreased rapidly to a negligible value as the alkyl concentration decreased. The apparent activation energy was 8.1 kcal./mole. The rate of polymerization with VCl 3 as catalyst component was generally faster than the rate obtained with TiCl 3 . A reaction scheme has been postulated to account for the influence of monomer and catalyst on the steady state rate. The model suggests that TiCl 3 activation consists of surface alkylation and removal of Cl − ions by reaction with Al(C 2 H 5 ) 3 . Monomer adsorption occurs on vacant sites, neighboring alkylated surface sites, in competition with the adsorption of metal alkyl. Vacant and alkylated sites partake in the first coordination sphere of Ti +3 ions in TiCl 3 . Polymerization follows by monomer insertion into the Ti[bond]C bond at the alkylated site.