Composition of (a) at least one strained cycloolefin and (b) a catalytic quantity of at least one divalent-cationic compound of ruthenium or osmium with a metal atom, to which are bound 1 to 3 tertiary phosphine ligands with, in the case of the ruthenium compounds, sterically exacting substituents, optionally non-photolabile neutral ligands and anions for charge balancing, with the proviso that in ruthenium (trisphenylphosphine)dihalides or hydride-halides the phenyl groups are substituted by C.sub.1 -C.sub.18 alkyl, C.sub.1 -C.sub.18 haloalkyl or C.sub.1 -C.sub.18 alkoxy. The composition is suitable for the production of molded parts of all kinds and of coatings.