A catalyst system for the polymerization of cyclic olefins, comprising(1) a tungsten compound which is obtainable by combining a tungsten halide and/or a tungsten oxyhalide with a phenol compound which is substituted with a bulky alkyl group at the positions ortho with respect to the hydroxyl group;(2) one or more compounds of the general formula ##STR1## wherein R.sup.1, R.sup.2 and R.sup.3 each represent an optionally substituted alkoxy group in the range of from 2 to 8 carbon atoms and preferably from 2 to 4 carbon atoms, and wherein at most one of the symbols R.sup.1, R.sup.2 and R.sup.3 may represent hydrogen or halogen and preferably chlorine, the compound of formula I being substantially free of organo-tin compounds;(3) a boron halide,and a process for in mould polymerization using such catalysts.