Abstract: A transition metal compound containing at least one .pi.-bonded arene is contacted with an inorganic oxide, hydroxide, oxyhalide, hydroxyhalide or halide. The transition metal compound can be a compound such as titanium(O)ditoluene or TiCl.sub.2.2AlCl.sub.3 durene. The inorganic compound is preferably of high surface area and this may be inherent in the compound or can be achieved by a grinding procedure. The inorganic compound can be treated with a halogen containing compound either before, during or after the contacting with the transition metal compound. The product of the contacting is useful either as catalyst for the polymerization of unsaturated hydrocarbon monomers or as a component of such a catalyst. When used as a component of an olefine polymerization catalyst, the transition metal product can be mixed with an organo-aluminium compound and a Lewis Base compound such as an ester.