Semi-fluidized bed hydrodynamics, including minimum and maximum semi-fluidization velocities, packed bed formation and pressure drop across the bed, have been studied in a 0.1 m ID liquid-solid semi-fluidized bed. The top grid is a specially designed perforated plate to have negligible pressure drop across it and to retain all the particles used in the experiment. Experimental parameters studied included velocity, density and viscosity of the liquid, particle size, static bed height and expansion ratio. Empirical and semi-empirical models have been derived. The results have been compared with those available in the literature. (C) 2009 Elsevier B.V. All rights reserved.