The scaling down of trickle bed reactors for catalyst testing in deep hydrodesulphurisation (HDS) is evaluated. A multiphase micro-reactor system has been built specifically for HDS, consisting of a set of six 2 mm diameter packed beds with particles of approximately 100 mu m. To confirm plug-flow behaviour (for integral conversion) and to guarantee the measurement of true kinetics, the hydrodynamics have to be investigated. For this purpose, a 'cold-flow' set-up of the same dimensions as the HDS reactors, has been built. A liquid feed with a dye tracer pulse as well as gas are fed to a glass column, packed with glass particles. From the recorded outlet concentration, the influence of the gas- and liquid-flow rate on the mean residence time and residence-time distribution (RTD) have been determined. This hydrodynamics investigation describes the deviation from plug flow in micro-scale packed beds. The results show that the deviations from plug flow are minimal. The effect of the gas-flow rate on the liquid-residence time is more pronounced in micro-packed beds than that in trickle beds with larger particles. The RTD study presented here provides valuable insight into the behaviour of scaled-down kinetic-test facilities. (c) 2005 Elsevier B.V. All rights reserved.