The reaction of cycloolefins with the components of the catalytic system WCl6/epichlorohydrin/Al(iBu)3, active in the ring-opening polymerization of cycloolefins, was investigated in order to get additional information on the mechanism of polymerization. The study led to the following conclusions. 1) Olefin complexation with the transition metal has a decisive influence on active center formation. 2) Trivalent tungsten species resulting from fast reduction of the cycloolef in/transition metal complex by aluminum alkyl or from a slow reduction process d W(VI) in the presence of cycloolef in alone, show a particularly high activity in ring-opening polymerization. 3) In the case of tricomponent systems (e.g., WCl6/Al(iBu)3/ epichlorohydrin or WCl6/Al(iBu)3/chloranil), the nature of the third component has a decisive effect on both polymerization kinetics and the molecular weight and structure of the resulting polymer. The data are discussed in the context of a reaction mechanism based on carbene complexes as active centers.