Polymerization process, in particular for the polymerization of monomer selected from ethylene and propylene in a reaction system containing at least three reactors operated in series. The process includes (a) polymerizing monomer in a first reactor having a volume of at least 50 m3 to produce a first polymer, (b) passing the first polymer to a second reactor and polymerizing monomer in the second reactor to produce a second polymer containing the first polymer and polymer produced in the second reactor, and (c) passing the second polymer to a third reactor and polymerizing monomer in the third reactor to produce a third polymer containing the second polymer and polymer produced in the third reactor. The polymerizations are performed such that between 0.01 and 5 w % of the total mass of the third polymer is polymer produced in the second reactor.