The present invention relates to a catalyst system for the production of polyethylene comprising: I) the reaction product obtained by reacting a) a hydrocarbon solution comprising: i. a magnesium-containing compound selected from an organic oxygen-containing magnesium compound and/or a halogen-containing magnesium compound; and ii. an organic oxygen-containing titanium compound wherein the molar ratio of magnesium:titanium is lower than 3:1; and b) an organo aluminium halide having the formula AlRnX3-n in which R is a hydrocarbon moiety containing 1-10 carbon atoms, X is a halogen and 0