AbstractCirculating Fluidised Beds (CFBs) are widely applied in the process industry, for mostly gas-solid and gas-catalytic reactions. The riser is the key component of the CFB being the process reactor. The important design parameters are the operating gas velocity (U) and the solids concentration flux (G). The CFB operation starts at moderate to high superficial gas velocities. Its voidage exceeds ∼ 0.9 and is a function of the solids circulation flux. Different flow modes have been presented in literature, and result in an operation diagram where G and U delineate specific operations, from dilute riser flow, through core-annulus flow, to dense riser upflow (mostly at any U, G exceeding 80 to 120 kg m-2s-1). Increasing G whilst maintaining the gas velocity will cause an increase in suspension concentration. The riser flow can hence be characterized by its apparent voidage, ɛ. In the core-annulus operation, clusters of particles reflux near the wall, thus influencing the local radial voidage in the cross section of the riser, and also extending over a given distance, δ, from the wall to the core .Through measurements in CFBs of 0.1 and 0.14 m I.D., the research has been able to determine the average axial and radial voidages of the dense phase within the different regimes, whilst also determining the thickness of the annulus (in CAF-mode). Experimental results will be illustrated and compared with previous empirical equations, shown to have a limited accuracy only both for ɛ, and for the thickness of the annulus in CAF operation. Within the operating conditions tested, results demonstrate that the annulus thickness is about 15 to 20% of the riser diameter in CAF, and that the voidage in the riser is a function of U and G, with riser diameter and distance along the riser length as secondary parameters.