Experiments were conducted to investigate the unsteady state void fraction characteristics of inverse fluidized bed (IFB) subjected to step change in liquid velocity. The resulting temporal and spatial evolution of void fraction in IFB is studied for the first time in the literature. The experimental profiles could be explained by the existing theories. The particle bed model proposed by Foscolo and Gibilaro [1987. Fluid dynamic stability of fluidized suspensions: the particle bed model. Chemical Engineering Science 42(6), 1489-1500.] is used to predict the experimental spatio-temporal variation of void fraction, and the time history of bed surface and voidage interface. In general, a good agreement is obtained between the experimental results and model prediction. The hyteresis phenomenon occurring during unsteady state conditions is also explained. (c) 2006 Elsevier Ltd. All rights reserved.