Beds of two granular materials belonging to groups A-B and B of the Geldart classification of powders were fluidized by carbon dioxide at 35 degrees C and at 1 to 80 bar. Within this pressure interval, spanning from subcritical to supercritical conditions, the fluid density changes from the values typical of dilute gases to those of liquids. The hydrodynamic behavior of the bed changes accordingly. The bed behavior was investigated by visual inspection of the bed and by analysis of the time series of the pressure drop across the bed and of the heat-transfer coefficient between the bed and a hot wire probe. The boundaries between different captive fluidization regimes-incipient fluidization, onset of bubbling, and the incipient and fully established turbulent regime - were mapped in gas superficial velocity us. fluid density phase planes for the two granular materials investigated.