While most catalytic fixed bed gas-liquid reactors of the petrol industry work at quite high pressures, the academic scientific work in this field concerned itself almost exclusively with the domain of approximatively atmospheric pressures. The authors present the results of some years of experimental investigations on the hydrodynamic characteristics of trickle bed reactors and lately of cocurrent upflow reactors. During the last years, results were also obtained under pressures up to 8 MPa. The measurements were made in a small scale cold flow equipment (diameter 23 mm). Different aqueous and organic more or less viscous, eventually coalescence inhibiting liquids, four gases and a number of non porous more or less wettable particles were used. The liquid holdup was determined in all cases by measuring liquid phase residence time distribution by different tracers. The following conclusions may be drawn:(a) In the high interaction regime, it is the inertia of the gas and the liquid phases which is the main cause of the dissipation of mechanical energy. In this regime, results obtained in cocurrent upflow and downflow are approximately equal. (b) Most correlations of literature are unable to predict the effect of pressure on the pressure drop or the liquid holdup. (c) The gas viscosity has no influence on the hydrodynamics. It is therefore possible to simulate for example hydrogen under high pressure conditions by another gas of the same density (at a much lower pressures). A critical evaluation of the correlations and/or models of literature is presented, concerning their ability to represent the different characteristics as a function of pressure.