A comprehensive experimental study of the hydrodynamics of gas-solid turbulent fluidized beds is reported for fluid cracking catalyst (FCC) particles in columns of diameter 0.26, 0.61 and 1.56 m. The flow dynamics were investigated by means of pressure transducers, optical probes and capacitance probes. The superficial gas velocity, U-c, at which the onset of the turbulent fluidization flow regime occurs, was found to depend on the aspect ratio, H/D. A new correlation, which accounts for the H/D influence, is proposed for predicting U-c. The bed expansion depends on the configuration of the solid collection and return system. The normalized radial profile of time-mean voidage was affected less by column diameter than by the normalized superficial gas velocity, U/U-c. A recently-developed optical probe capable of simultaneously measuring particle velocity and voidage delineated the change in local two-phase flow structure when the superficial gas velocity exceeded U-c. Local measurements confirm that the two-phase flow structure breaks down once U>U-c; however, the extent of this breakdown depends on the column diameter. (C) 2004 Elsevier B.V. All rights reserved.