Titanium tetrachloride heterogenized on reduced TiO2 has been studied as a catalyst for ethylene polymerization. The catalyst has good storage stability and exhibits good activity for ethylene polymerization. The polymer chains grow linearly during ca. 1 h, giving an average molecular weight of up to 2.5 X 10(6) which indicates that practically no beta-elimination occurs. The activity of the catalyst at 50-degrees-C, based on Ti(III), is 7.6 X 10(6) PE/mol Ti h bar and based on the quantity of polyethylene formed it is 1.25 X 10(6) g PE/mol Ti h bar. The molecular weight of the polymer can be controlled with the addition of hydrogen; under 0.5 bar hydrogen, polyethylene with a molecular weight of 411,000 and a relatively low polydispersity index of 2.2 is obtained. The catalyst shows good thermal stability; the Arrhenius activation energy is 31.8 kJ/mol for the polymerization. The catalyst is also active for propylene polymerization, giving 3 X 10(6) g PP/mol Ti h bar with the high isotacticity of 93%.