Hydrodynamic transition experiments, involving both visual observations and pressure measurements, were performed using a 127-mm diameter Plexiglas column for three-phase inverse fluidized beds of 5.8-mm polyethylene spheres. Observations of interest not hitherto reported include: (1) A marked hysteresis effect (even when starting from a loose-packed condition) between inverse fluidization and defluidization which disappears when a wetting agent is added to the downflowing water. (2) An initially abrupt decrease of the minimum fluidization voidage, epsilon (mf), followed by a gradual rise of epsilon (mf) with increasing superficial gas velocity U-g. (3) Lower values of epsilon (mf) for three-phase systems than for the corresponding two-phase (liquid-solid) fluidized beds because local agitation by the gas bubbles causes bed compaction near the minimum liquid fluidization velocity, U-lmf (4) U-lmf vs. U-g curves which, though they always show U-lmf decreasing as U-g increases, sometimes display concave-downward, sometimes concave-upward and sometimes S-shaped behavior.