Abstract: A process for the polymerization of one or more conjugated dienes by means of a rare earth based catalyst in which the catalyst is formed by bringing together in liquid hydrocarbon medium in the presence of a small portion of conjugated diene: a) a rare earth salt, b) an organo aluminum and c) a silicon halide or organosilicon halide and aging the catalyst before contact with the conjugated diene(s) to be polymerized. This catalyst formation procedure offers the possibility of producing a diene polymer of high cis content with a narrower molecular weight distribution and/or smaller high molecular weight fraction which is reflected in improved processing characteristics and reduced viscosity in styrene.