Experiments were performed to study the hydrodynamic characteristics of ebullate-bed systems operated under conditions of high pressure and temperature. The effects of these variables on bed porosity and the liquid minimum fluidization velocity were determined and a correlation has been proposed as a criteria for determining flow regime transitions between the dispersed and coalesced bubble flow regimes. The bed porosity has been described with the pseudo-fluid model. The minimum fluidization velocity data were contrasted with predictions from empirical correlations reported in the literature. (c) 2005 Elsevier B.V. All rights reserved.