The focus of this work is the determination of the flow structure which gives rise to the S-shaped profile of axial pressure gradient characteristic of a circulating fluidized bed riser operating at fast fluidization conditions. The flow structure is characterised by measurement of radial profiles of solids momentum and solids volume fraction in a riser 0.09 m in diameter and 7.2 m high using a momentum probe and capacitance tomography. The solids used were sand with a Sauter mean diameter of 100 mu m and particle density of 2650 kg/m(3). The core-annulus suspension structure typically found in the upper dilute region of the fast bed is found to extend throughout the S-shaped profile and into the lower dense region. However, the direction of solids Bow in the annulus changes from downwards in the upper dilute region to upwards in the lower dense region. The interface between the regions (the middle of the S) is found to be a region of relatively low axial solids momentum throughout the riser cross-section. The fast fluidized bed structure suggested by these observations is presented in the form of a diagram.