High-velocity gas fluidization includes the turbulent- and fast-fluidization regimes which lie beyond the low-velocity bubbling or slugging regimes. Bed expansion and solid mixing were studied in 15.2-cm ID rigs, of high aspect ratios. Fine cracking catalysts of Geldart's group A classification were used and air used at 0.075 to 6.5 m/s, solid rate in the upper end of the fast fluidization regime topped 150 kg/m/sup 2/.s. The Two-Phase Theory does not apply when the gas velocity is raised beyond the slugging regime: the bed structure becomes more homogeneous. Particulate-like fluidization describes these high-velocity regimes. A modified Richardson-Zaki approach is used to correlate bed expansion. It is shown to confirm the more homogeneous nature of the turbulent and fast regimes, and the validity of the concept of clusters - or the tendency of fine particles to aggregate. Solid mixing was studied with a ferromagnetic tracer and modeled by a turbulent eddy-diffusivity dispersion model. Implications for heat transfer in a high-velocity fluidized bed are discussed.