Abstract: Polymerization of ring-strained cyclic olefins takes place in the presence of a one-part or two-part catalyst which is air and moisture stable and which comprises a transition metal-containing compound.The one-part transition metal-containing catalyst is selected from the group consisting of(a) compounds of the formula(L.sup.1)(L.sup.2)QwhereinQ represents Mo or W;L.sup.1 represents one to six CO (carbonyl) ligands;L.sup.2 represents none to 5 ligands, each of which is a non-ionic compound or unit of a polymer which can be the same or different, each contributes two, four, or six electrons to the metal and is different from L.sup.1 ; wherein the sum of the valence electrons of Q and the electrons contributed by the ligands L.sup.1 and L.sup.2 is 18,(b) at least one of cationic ruthenium and osmium-containing organometallic salts having at least one polyene ligand, and(c) [Ir(RO.sub.2 CHC.dbd.CHCO.sub.2 R).sub.2 Cl].sub.2 wherein each R independently is hydrogen or lower alkyl (C.sub.1 to C.sub.