The influence of fluid dynamics on the hydrodesulfurization (HDS) reactions of a diesel oil in bench-scale reactors was evaluated. The porosities and liquid saturations of catalyst beds were quantified by using the MRI technique. The gas-liquid systems used in the experiments were nitrogen diesel and hydrogen diesel. An apparatus was especially constructed, allowing in situ measurements of gas and liquid distributions in packed beds-at elevated pressure and temperature up to 20 bar and 200 degrees C, respectively. The reactor itself had a length of 500 mm and an internal diameter of 19 mm. The packed beds used in this MRI study consisted of: (1) 2 mm diameter nonporous spherical glass beads and (2) 1.3 mm diameter porous Al2O3 trilobes having the same size as the original trilobe catalyst used in HDS bench-scale experiments. The superficial gas and liquid velocities were set within the range of trickle flow, e.g., u(0G) = 20-500 mm/s and u(0L) = 0.1-6 mm/s. In parallel with the MRI experiments, the hydrodesulfurization of a gas oil was investigated in a bench-scale plant. Its reactor had the same dimensions of the trickle-bed column used in the MRI experiments and was filled with original trilobe catalyst. These catalytic experiments were carried out at a wide range of operating conditions (p = 30-80 bar, T = 300-380 degrees C, LHSV = 1-4 h(-1)). The results of both fluid dynamic and catalytic reaction experiments were then combined for developing a simulation model to predict the HDS performance by accounting for fluid dynamic nonidealities.