The objective of the present work is to determine the influence of the two-phase flow direction in hydrotreating bench scale plants on the axial dispersion of the liquid phase. Residence time distribution experiments under ambient conditions in cocurrent upflow and in cocurrent downflow are carried out in a catalytic hydrotreating bench scale plant, which contains a thermowell. Particles that are representative of a commercial catalyst are used. The fluids and flow rates are chosen in order to simulate deep desulfurization conditions of a straight-run gas oil. In order to determine the axial dispersion only within the bed of particles, a radioactive tracer is used. The hydrodynamic parameters are identified using an axial dynamic piston dispersion model. The liquid axial dispersion is found to be significantly higher in the downflow mode than in the upflow mode. The values of the upflow liquid saturation are in a good agreement with the values found in the literature whereas the downflow liquid saturation is lower. Simulations with a multiphase model indicate that the difference of the axial dispersion might have a significant influence on the hydrodesulfurization performances.