Hydrodynamic experiments were performed using a 127-mm diameter column with 3.2-mm porous alumina, 3.3-mm polymer blend, 5.5-mm polystyrene and 6.0-mm glass spheres, with water, aqueous glycerol solution and silicone oil as liquids, and air as the gas. The voidage at minimum fluidization fell initially to a minimum, then rose gradually with increasing superficial gas velocity, and was lower for three-phase systems than for corresponding two-phase (liquid-solid) fluidized beds. The compaction appears to be due to agitation by gas bubbles near the minimum liquid fluidization condition. The gas holdups agree reasonably well with the correlation of Yang et al. (1993). Curves of minimum liquid fluidization velocity, U-Imf, vs. superficial gas velocity, U-g., always show U-Imf decreasing as U-g increases, initially in a concave-downward manner, but sometimes concave-upward.