Abstract: The present invention relates to a continuous process for controlling the gas-phase co-polymerisation of olefins in a fluidised bed reactor. The present invention further relates to a method for the continuous gas-phase (co-)polymerisation of olefins in a fluidised bed reactor in the presence of a polymerisation catalyst wherein the density of the polymer powder particles leaving the reactor is maintained constant by controlling the particle size distribution of the polymerisation catalyst. The present invention also relates to a process for controlling the particle size distribution of a polymerisation catalyst in order to achieve the production of a polymer powder of constant density during the continuous gas-phase (co-)polymerisation of olefins in a fluidised bed reactor.