A continuously operated and computer controlled plant for high-pressure reactions and separations, utilising pressures up to 350 MPa, was designed and constructed. The technical characterisation of the plant is based on the flow-through behaviour. Thus, breakthrough curves were recorded at pressures of 0.8, 20, and 300 MPa, imposing a step input signal with methylene blue as tracer substance in aqueous solution. Hydrodynamic plant parameters as mean times, variances, the number of theoretical plates, Bodenstein number, and axial dispersion coefficient were determined, utilising the tanks-in-series and the dispersion model. At first, all experiments were carried out without a fixed bed in order to act the system's flow parameters. After that, a typical fixed bed consisting of modified silica gel that adsorbed almost no dye was used. These investigations were carried out to separate pore diffusion, mass transfer, and adsorption influences from disturbance caused by the fixed bed. At last, a highly adsorbing silica gel was applied to simulate a complete adsorption process. The results show that there is nearly no pressure dependency of the system's flow parameters and the disturbance, respectively. Adsorption onto silica gel, which is strongly influenced by pore diffusion and mass transfer, shows a significant pressure dependence. Mass transfer coefficients decrease with increasing pressure and so limit any desired surface reaction. Furthermore, for the amount of dispersion at hand, the number of theoretical plates N always exceeds Bol2; consequently some standard equations for the determination of dispersion are not applicable. (C) 2004 Elsevier B.V. All rights reserved.