Process for the polymerization of olefins in a gas phase fluidized bed reactor, by i) passing a fluidizing gas to the reactor to fluidize a bed of polymer particles therein, ii) withdrawing the fluidizing gas from the reactor, iii) cooling all or a portion of the fluidizing gas withdrawn from the reactor to a temperature at which a portion of the components of the fluidizing gas condense to form condensed liquid, and iv) passing condensed liquid and non-condensed components of the fluidizing gas back to the reactor. A reservoir of condensed liquid downstream of the cooling step is provided, and variations in the inventory of the condensed liquid are made in order to control the temperature in the reactor.