Sieved glass beads of 230, 280 and 470 mu m dia. were steadily suspended by upward concurrent gas and liquid flows in vertical bubble columns of 7 cm dia. and 4.85 m height or 15 cm dia, and 2.7 m height. Then the liquid velocity was suddenly increased to a higher value which caused an unsteady-state discharge of solid particles from the top of the column. The transient discharge rate was determined by collecting solid particles in the effluent, and transient axial distributions of gas and solid holdups along the column length were measured by sectioning the column with horizontal shutter plates just after stopping gas and liquid flows. A 1-D sedimentation-dispersion model was applied to describe the unsteady-state discharge rate of solid particles as well as the change in gas and solid holdups in the columns. The model developed successfully expressed the a transient behavior of solid particles over entire columns consisting of dense and dilute regions.