The hydrodynamics and flow regime in fast fluidized beds of FCC and silica sand particles were determined in a Plexiglas column (0.1 m i x 5.3 m high). The transport velocity (U-tr) was determined by the emptying time and flooding point determination methods. At a given solid circulation rate, solid holdups in the upper dilute region exhibit their maximum values with variation of gas velocities, where a dense region forms at the bottom of the riser. The gas velocity at which solid holdup in the upper dilute region exhibits a maximum value is defined as the fast transition velocity (U-FT) The fast fluidization regime can be divided into the fast transition region and the fully developed fast fluidization region. A demarcation Line between the two regions is proposed. The fast transition velocity and the transition velocity (U-FD) to pneumatic transport have been correlated. The transition velocity of the fast fluidization region can be predicted by the choking velocity, U-FD and U-FT.(C). 1999 Elsevier Science S.A. All rights reserved.