Abstract: The present invention relates to an apparatus and a process for polymerization, and, in particular, provides an apparatus for gas phase fluidised bed polymerization of olefins, which apparatus comprises: A) a first section which is an upright cylindrical section having a diameter, D1, and cross-sectional area, A1, and B) a second section, provided vertically above the first section and centered about a common vertical axis to the upright cylindrical first section, the base of the second section having a cylindrical cross-section of diameter D1 and being joined to the top of the first section, and the horizontal cross-sectional area of the second section above its base being greater than the cross-sectional area of the first section, characterized in that: i) D1 is greater than 4.5 meters, and ii) the second section has a maximum horizontal cross-sectional area, A2, which is between 3.2 and 6 times the cross-sectional area, A1, of the first section.