A process for the preparation of a catalyst for the polymerization and copolymerization of cyclic olefins which have 4, 5, 7 or more ring carbon atoms and at least one olefinic double bond in the ring, fromA. at least one halide or oxyhalide of molybdenum, preferably with molybdenum in the oxidation state +5,B. at least one 2-halo alcohol andC. at least one aluminium compound in the molar ratio of a : b = 1 : 1 to 1 : 10 and a : c = 1 : 0.5 to 1 : 30,Characterized in that the molybdenum halide or oxyhalide and the 2-halo alcohol are reacted with the exclusion of solvents or in the presence of chlorinated solvents which are inert towards the molybdenum salt and in which the molybdenum salt is soluble to an extent of at least 0.01 molar, and the aluminium alkyl compound is added, optionally in the presence of the monomers.