Process for polymerizing ethylene in a reactor system containing one or more reactors in series, having a characteristic such that the average polymerization productivity [kgPE/kgcata] per unit ethylene per hour a1 during operation at any first residence time r1 is less than 1.7 (a2r2−a1r1)/(r2−r1), where a2 is the average polymerization productivity [kgPE/kgcata] per unit ethylene per hour during operation at any second residence time r2 where r2>r1, a2 and r2 being measured either in the same reactor in the case of a single reactor polymerization, or in a reactor subsequent to the reactor in which a1 and r1 are measured in the case where the polymerization takes place in more than one reactor. The specific yield of the reactor system is greater than 0.3 tonnes/m3. Operating the system under the above conditions results in improved productivity/unit ethylene.