Choking is commonly defined as a phenomenon where a sudden change in the solids holdup occurs in a gas-solid fluidization system. In this study, the electrical capacitance tomography (ECT) based on the neural network multicriteria optimization image reconstruction technique (NNMOIRT), developed by this research group, is used to probe the mechanism of choking formation by examining a real-time, quasi-3D cross-sectional flow structure of a circulating fluidized bed riser. The particles used are FCC catalysts. The ECT with NN-MOIRT reveals a double solids-ring flow structure and the presence of particle blobs at the center of the bed in the circulating fluidized bed (CFB) riser. This flow structure, when it is present at the entrance region of the riser, undergoes a distinct variation during the choking transition when the gas velocity is below the transport velocity U-tr, but does not undergo a distinct variation when the gas velocity is above U-tr, This flow structure, when it is present at the upper region of the riser, however, does not vary distinctly at any gas velocity. Such flow structure variations are not appreciably feeding patterns with or without a gas or solids distributor, or by the affected by the solids gas humidity. For the low gas velocities (< U-tr), the borescope measurement indicates that the solids concentration in the particle blob and the blob size all increase as the solids circulation rate increases. The disintegration of the enlarged blobs or blob jets and the collapse of the solids suspension characterize the mechanics of the initiation of the choking transition to the dense-phase fluidization regime attributed to solids suspension instability. (C) 2004 American Institute of Chemical Engineers.