G as-liquid-solid three-phase fluidized beds have gained considerable importance as shown in their commercial applications, such as catalytic cracking processes, Ficher-Tropsch synthesis, hydrometallurgical operations, coal liquefaction and aerobic and anaerobic treatment of wastewater (Fan, 1989). These successful applications lie in comprehensive understanding of the three-phase fluidization system, including hydrodynamics, heat and mass transfer and mixing. Extensive studies have been carried out in the conventional three-phase fluidized beds where particles are fluidized in the bed and the bed height is well defined within the system (Chen et al., 1995;Matsumoto et al., 1997). However, conventional three-phase fluidized beds are not suitable to apply to the strong exothermic systems, systems employing light/small particles as well as the systems requiring catalysts to be regenerated. Gas-liquid-solid three-phase circulating fluidized bed (GLSCFB) is one specific type of three-phase fluidized beds with outer particle circulation, which is efficient in numerous multiphase industrial applications because of good phases contact, excellent heat and mass transfer characteristics and highoperated flexibility (Liang et al., 1997) Despite the distinct advantages of GLSCEB, few studies have been documented. Liang et al. (1997) first studied the hydrodynamic characteristics of the three-phase circulating fluidized bed using the conductivity probe. It was reported that the gas and solid holdups were distributed uniformly along the riser, whereas, the radial distributions were non-uniform. These findings agree with Yang et al. (1999).There is no report focusing on the effect of operating conditions and particle size on the flow structure. In order to provide an efficient design and operation of the GLSCFB, its hydrodynamic behaviour must be studied and understood, as well as the influence of the operating conditions.In this study, an ultrasonic technique has been applied to simultaneously measure phase holdups. In a multiphase system when an acoustic wave strikes the boundary between two different media and the acoustic impedances of the two media differ, some acoustic energy is reflected and some is transmitted. The reflected wave travels back through the incident medium at the same velocity. The transmitted wave continues to move through the new medium at the sound velocity of the new medium. Since the sound wave is partially scattered and absorbed while traveling in different media, the amplitude of the sound wave is reduced accordingly. An accurate measurement of the variations in sound speed The axial and radial distributions of solid and gas holdups were investigated in an air-water-glass bead circulating fluidized bed (GLSCFB) using a new ultrasonic technique, with a new method based on signal fluctuations. The cross-sectional averaged gas and solid holdups measured at two axial positions appear to be similar at all studied operating conditions. The radial non-uniformity decreases with increasing liquid velocity bu...