Abstract: A supported Ziegler catalyst prepared by reacting, under substantially anhydrous conditions, a refractory oxide support material having acidic surface hydroxyl groups, e.g. silica, and an organo metallic compound, e.g. a trialkyl aluminium compound, separating any unreacted organo metallic compound and impregnating the solid product with a halogen-containing titanium compound and a halogen-containing vanadium compound. The catalyst, preferably together with a conventional Ziegler catalyst activator, e.g. an aliminium alkyl, is employed to polymerize 1-olefins, for example ethylene.