The pressure-velocity hysteresis phenomenon in gas-solids conical spouted beds was investigated based on pressure measurement and visual observation in a semicircular column. Experimental data in combination with a mechanistic model revealed that the hysteresis is primarily caused by different bed structures resulting from different compressed states of particles in the vicinity surrounding the internal spout or cavity, due to different interlocking behavior of particles in the upper vicinal packed-bed region in ascending and descending process. The height of the internal spout in the velocity ascending process was significantly shorter than the descending process due to the increased resistance to the gas jet penetrating through the compressed region. (c) 2005 Elsevier Ltd. All rights reserved.