Flow regime boundaries and pressure drop in trickle bed reactors are crucial for design, scale-up and operation of such reactors. The flow map experiments are performed in a pilot plant reactor of 0.051 m diameter and 1.2 m height, with cumene-hydrogen system. A new technique - the acoustic signal measurement - is used for distinguishing between trickle and pulse flow regimes. The effect of operating pressure was investigated in the pressure range of 0.14-2.0 MPa. For higher operating pressures, the trickle-to-pulse transition boundary moves towards higher flow rates of both liquid and gas phases. The pressure drop over the reactor bed is increasing with increasing operating pressure and gas/liquid throughputs. The pressure drop results obtained with hydrogen at higher operation pressures match reasonably well the results obtained with air-water at atmospheric pressure. This comparison is made using a new developed pressure drop correlation and illustrates the influence of increased gas density (high operating pressure effect). The Trickle Bed Simulator of University Laval [F. Larachi, B. Grandjean, I. lliuta, Z. Bensetiti, A. Andre, G. Wild, M. Chen, Excel Worksheet Simulator for Trickle-Bed Reactors, http://www.gch.ulaval.cafbgrandjean/pbrsimul/pbrsimul.html, 1999] was found to match reasonably well our pilot plant measured values for low and high operating pressures. (c) 2005 Elsevier B.V. All rights reserved.