A control method for controlling a fluidized bed polymerization reactor in the production of a given polymer product, the method comprising the following steps: (a) determining a ratio of the production rate of the polymer product in the reactor to the pressure in the reactor, (b) setting a production rate of the polymer product in the reactor which production rate, on the basis of said ratio of step (a), corresponds to a desired pressure in the reactor; (c) adjusting the feed rates of monomers into the reactor in accordance with said set point production rate.