A process for the modification of unsaturated polyolefin rubbers by reaction with other polymers amenable to the metathesis reaction, in the liquid phase, preferably in solution, in the presence of a metathesis catalyst system comprising (a) a soluble compound of a metal of the Subgroups V through VII of the periodic table, and (b) a soluble alkyl or hydride of a metal of Main groups I through III of the periodic table, as well as optionally activators, characterized by the metallic components of the catalyst system each being dispersed, preferably dissolved, separately in partial streams wherein the reaction components are contained, and these partial streams are intermixed intensively.