Abstract: The present invention relates to a process for stopping and restarting polymerisation in a polymerisation system comprising a gas phase fluidised bed or stirred bed reactor, a degassing vessel and a seedbed storage silo, said process comprising the steps of: a) Conducting a first polymerisation reaction in the reactor using a first polymerisation catalyst system to produce a first polymer, b) Stopping the first polymerisation reaction and removing at least the majority of the first polymer in the reactor to the degassing vessel, and either: c1) Introducing from the seedbed storage silo and into the reactor a seed-bed compatible with a second polymerisation reaction, said seed-bed being of a different material to the first polymer, d1) Passing to the seedbed storage silo at least a portion of the first polymer from the degassing vessel to thereby form a new seedbed in the storage silo, and e1) Conducting the second polymerisation reaction in the reactor to produce a second polymer; or c2) Optionally, maintaini