A composition, comprising (a) a compound of formula I (R1R2R3P)xLyM2+Z1−Z2−   (I), wherein R1, R2 and R3 are each independently of one another H, C1-C20alkyl, C1-C20-alkoxy; C4-C12cycloalkyl which is unsubstituted or substituted by C1-C6-alkyl, C1-C6haloalkyl or C1-C6alkoxy; C4-C12cycloalkoxy which is unsubstituted or substituted by C1-C6-alkyl, C1-C6haloalkyl or C1-C6alkoxy; C6-C16aryl which is unsubstituted or substituted by C1-C6alkyl, C1-C6haloalkyl or C1-C6alkoxy; C6-Cl6aryloxy which is unsubstituted or substituted by C1-C6alkyl, C1-C6haloalkyl or C1-C6alkoxy; C7-C16-aralkyloxy which is unsubstituted or substituted by C1-C6alkyl, C1-C6haloalkyl or C1-C6alkoxy; or C7-C16aralkyloxy which is unsubstituted or substituted by C1-C6alkyl, C1-C6-haloalkyl or C1-C6alkoxy; or R2 and R3 together are tetra- or pentamethylene which is unsubstituted or substituted by C1-C6alkyl, C1-C6haloalkyl or C1-C6alkoxy; tetra- or pentamethylenedioxyl which is unsubstituted or substituted by C1-C6alkyl, C1-C6haloalkyl or C1-C6alkoxy; tetra- or pentamethylene which is condensed with one or two 1,2-phenylene radicals and which is unsubstituted or substituted by C1-C6alkyl, C1-C6haloalkyl or C1-C6alkoxy; tetra- or pentamethylenedioxyl which is condensed with one or two 1,2-phenylene radicals and which is unsubstituted or substituted by C1-C6alkyl, C1-C6haloalkyl or C1-C6alkoxy; or tetramethylenedioxyl which is condensed in the 1,2- and 3,4-positions with 1,2-phenylene and which is unsubstituted or substituted by C1-C6alkyl, C1-C6haloalkyl or C1-C6alkoxy; and R1 has the meaning cited above; L is a neutral ligand; M is Ru or Os; Z1− and Z2− are each a singly charged anion, or Z1− and Z2− 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≦x+y ≦4; and (b) a compound of formula II R4—C≡C—H   (II), wherein R4 is hydrogen or a monovalent organic radical of up to 30 carbon atoms which may contain one or several silicon, phosphorus, nitrogen, oxygen, sulfur or halogen atoms, 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 cydopentadiene adducts within a short time and with high enthalpy of reaction and only little weight loss.