A composition, comprising(a) a compound of formula I(R.sub.1 R.sub.2 R.sub.3 P).sub.x L.sub.y M.sup.2+ Z.sub.1.sup.- Z.sub.2.sup.- (I),whereinR.sub.1, R.sub.2 and R.sub.3 are each independently of one another H, C.sub.1 -C.sub.20 alkyl, C.sub.1 -C.sub.20 -alkoxy; C.sub.4 -C.sub.12 cycloalkyl which is unsubstituted or substituted by C.sub.1 -C.sub.6 alkyl, C.sub.1 -C.sub.6 haloalkyl or C.sub.1 -C.sub.6 alkoxy; C.sub.4 -C.sub.12 cycloalkoxy which is unsubstituted or substituted by C.sub.1 -C.sub.6 -alkyl, C.sub.1 -C.sub.6 haloalkyl or C.sub.1 -C.sub.6 alkoxy; C.sub.6 -C.sub.16 aryl which is unsubstituted or substituted by C.sub.1 -C.sub.6 alkyl, C.sub.1 -C.sub.6 haloalkyl or C.sub.1 -C.sub.6 alkoxy; C.sub.6 -C.sub.16 aryloxy which is unsubstituted or substituted by C.sub.1 -C.sub.6 alkyl, C.sub.1 -C.sub.6 haloalkyl or C.sub.1 -C.sub.6 alkoxy; C.sub.7 -C.sub.16 aralkyl which is unsubstituted or substituted by C.sub.1 -C.sub.6 alkyl, C.sub.1 -C.sub.6 haloalkyl or C.sub.1 -C.sub.6 alkoxy; or C.sub.7 -C.sub.16 aralkyloxy which is unsubstituted or substituted by C.sub.1 -C.sub.6 alkyl, C.sub.1 -C.sub.6 haloalkyl or C.sub.1 -C.sub.6 alkoxy;or R.sub.2 and R.sub.3 together are tetra- or pentamethylene which is unsubstituted or substituted by C.sub.1 -C.sub.6 alkyl, C.sub.1 -C.sub.6 haloalkyl or C.sub.1 -C.sub.6 alkoxy; tetra- or pentamethylenedioxyl which is unsubstituted or substituted by C.sub.1 -C.sub.6 alkyl, C.sub.1 -C.sub.6 haloalkyl or C.sub.1 -C.sub.6 alkoxy; tetra- or pentamethylene which is condensed with one or two 1,2-phenylene radicals and which is unsubstituted or substituted by C.sub.1 -C.sub.6 alkyl, C.sub.1 -C.sub.6 haloalkyl or C.sub.1 -C.sub.6 alkoxy; tetra- or pentamethylenedioxyl which is condensed with one or two 1,2-phenylene radicals and which is unsubstituted or substituted by C.sub.1 -C.sub.6 alkyl, C.sub.1 -C.sub.6 haloalkyl or C.sub.1 -C.sub.6 alkoxy; or tetramethylenedioxyl which is condensed in the 1,2- and 3,4-position with 1,2-phenylene and which is unsubstituted or substituted by C.sub.1 -C.sub.6 alkyl, C.sub.1 -C.sub.6 haloalkyl or C.sub.1 -C.sub.6 alkoxy; and R.sub.1 has the meaning cited above;L is a neutral ligand; M is Ru or Os; Z.sub.1.sup.- and Z.sub.2.sup.- are each a singly charged anion, or Z.sub.1- and Z.sub.2.sup.- together are a doubly charged anion; x is a number from 1 to 3; and y is a number from 0 to 3, wherein 2.ltoreq.x+y.ltoreq.4; and(b) a compound of formula IIR.sub.4 --C.tbd.C--H (II),wherein R.sub.4 is a monovalent organic radical of up to 30 carbon atoms which contains at least one hydroxyl group,is suitable as storage-stable two-component catalyst for the ring-opening metathesis polymerisation of cyclopentadiene adducts and makes it possible to polymerise dicyclopentadiene and other cyclopentadiene adducts within a short time and with high enthalpy of reaction and only little weight loss.