Apparatus and process for gas phase polymerisation of olefins. The apparatus includes (a) a reaction zone having a grid at its base, (b) an inlet located in the lower half of the reaction zone for introducing a reaction gas to the reaction zone, (c) an outlet located in the upper half of the reaction zone for removing the reaction gas from the reaction zone, and (d) a solids separation unit having an inlet fluidly connected to the outlet for removing the reaction gas from the reaction zone. The inlet of the solids separation unit is located at a vertical height lower than the outlet for removal of reaction gas from the reaction zone and such that the angle to the horizontal of a straight line drawn between the inlet of the solids separation unit and the outlet for removal of reaction gas from the reaction zone is greater than 20°.