Two pressure probes and an intrusive bioptical probe provide experimental data in the bubbling and turbulent regimes to improve our understanding of the transition front the bubbling regime to the fast fluidization regime. Assessment of pressure, voidage, and bubbling properties allows for a new description of turbulent fluidization hydrodynamics. Appreciable changes in. the hydrodynamics appear well below the transition criterion U-c, determined based on pressure fluctuations: bubbles are fast bubbles below U-c, in bubbleemulsion equilibrium state at U-c, and then become slow bubbles, for higher,fluidizing velocity; the maximum of the total fuidizing gas fraction in the bubble phase is reached below U-c and never exceeds 75-80%. The fluidizing velocity at which major modifications in hydrodynamics are observed is lower than U-c. This critical velocity is detected with pressure drop assessment or frequency analysis of voidage fluctuations. Moreover, it can be theoretically calculated from the two-phase modeling correlations supplemented with a new criterion deduced from our experiments. (c) 2005 American Institute of Chemical Engineers