For the polymerization of ethylene, as such or with one or more alpha-olefins, a novel catalyst composition is described, which gives a high yield of polymer per weight unit of catalyst. Such compositions are prepared from a titanium compound in which the valency of Ti is 3 or more, said compound being reacted with the vapors of one or more metals of the group consisting of Al, Cr, Mn, V, Ti, Zr, Mo, Zn and Ca in the presence of a halogen donor which is preferably an organic halide, certain inorganic halides being capable of being used provided that the halide metal has at least two degrees of valency. The composition is used in association with an aluminium hydrocarbyl halide. Yields as high as 30,000 grams of polymer per gram of elemental Ti can be obtained.