Method of changing from a polymerization using a first catalyst to a polymerization using a second catalyst which is incompatible with the first catalyst in a gas-phase reactor, which comprises the steps a) stopping of the polymerization reaction using the first catalyst, b) flushing of the reactor under polymerization conditions with at least one deactivating agent comprising a volatile constituent and a nonvolatile constituent in a weight ratio of from 0.1 to 1000, c) introduction of the second catalyst into the reactor and d) continuation of the polymerization using the second catalyst.