After having analyzed the few literature results concerning fluidized beds hydrodynamics under reduced pressure, new theoretical elements are proposed which provide an estimation of the beginning and finishing fluidization velocities. Numerous experimental results (U-mfapp, U-mb, U-mp, epsilon(mf), epsilon(mb), epsilon(mp)), obtained at 20 and 500 degrees C, are then presented for several sub-atmospheric pressures. Finally; for the first time, at least to our knowledge, by high frequency recording pressure drops through the bed, the influence of pressure decreases on the hydrodynamics is accurately analyzed.