Abstract: A process is described for the polymerization of ethylene or copolymerization of ethylene with a C.sub.3 to C.sub.15 alpha olefin in solution, suspension or in the gas phase at 20.degree. to 250.degree. C. and 1 to 100 bars in the presence of a mixed catalyst comprising a separately produced halogen-containing titanium(III) compound and an organoaluminum compound while controlling the molecular weight with oxygen, wherein a mixture of organoaluminum compounds is employed comprising a halogen-containing aluminum alkyl and a trialkyl aluminum compound in a molar ratio of 1:0.05 to 1:20, the combined amount of organoaluminum compounds being present in an amount of 0.01 to 20 moles per one mole of halogen-containing Ti(III) compound.