The gas-liquid-solid three-phase circulating fluidized bed (TPCFB) is a novel multiphase reactor with outstanding performance in hydrodynamics, heat transfer, mass transfer and catalyst treatment. The object of this work is to study the bubble behavior in TPCFBs for better understanding of the hydrodynamics of the reactor. The local gas holdup, bubble size distribution, and bubble rise velocity in different radial positions are measured using a fiber optic probe, and the effects of operating conditions on bubble rise velocity are also investigated. Moreover, the bubble rise velocity and its distribution are compared with the data from gas-liquid two-phase external loop airlift reactors and gas-liquid-solid three phase fluidized beds without external circulation of liquid and particles. (C) 2003 Elsevier B.V. All rights reserved.