Laboratory-scale trickle bed reactors are often used in testing of catalyst life time and activity. One of the important processes is represented by hydrodesulphurization (HDS) of motor fuels and petrochemical fractions. The study of stability of catalysts activity can be influenced both reactions kinetics, hydrodynamics and mass transfer phenomena. In case of ideal behaviour, the liquid could report piston flow. Non-ideal liquid flow can lead to incomplete catalyst particle wetting, which results in poor utilization of the catalytic surface of the particles. It may also expected to occur stagnant zones and imperfect heat transfer and overheating of the catalytic particles. For limitation of this negative phenomena should be diluted catalytic bed by small inert particle provide more contact area. The aim of this study was to determine degree of the effect of hydrodynamic conditions in model trickle bed reactors by residence time distribution method. Objective was to evaluate liquid holdup, axial dispersion and pressure drop of reactor bed consisting of catalyst particles diluted by inert fines to obtain data for process description by PD mathematical model. Hydrodynamic experiments were carried out in model glasses reactor with I.D 30.4 mm. Catalyst –trilobe extrudes of 1.3 mm O.D. was diluted by fine grain SiC 0.1-0.15 mm to obtained complete catalyst wetting and uniform liquid distribution. In presumed range of liquid and gas flow rates, air and water mass flow corresponded to values of hydrogen and hydrocarbons mixture in high-pressure reactor for the study of HDS catalysts. Gas flow was measured by MFC and liquid flow rate was checked by weighing, pressure drop was measured by pressure probe. Along the bed were located three electrodes to measure conductivity signal of responses to impulse of KCl solution. Data acquisition was made by four channel chromatography Clarity SW/HW, with subsequent evaluation by program in Matlab.