A three-phase reactor model for describing the hydrotreating reactions in a trickle-bed reactor was developed. It includes correlations for determining mass-transfer coefficients, solubility data, and properties of the compounds under process conditions. The model, based on the two-film theory was tested with regard to the hydrodesulfurization of vacuum gas oil in a new high-pressure pilot plant operated under isothermal conditions. The sulfur content of the product oil was found to depend strongly on the gels/oil flow ratio within the reactor. This is due to the inhibiting effect of hydrogen sulfide oil the chemical reaction rates described by Langmuir- Hinshelwood kinetics. The poor conversion which, in contrast to industrial plants, is often observed in pilot plant reactors can be explained by incomplete catalyst wetting produced by low liquid velocities. The simulation shows a good agreement with the experiments carried out in a wide range of temperature, pressure, space velocity and gas/oil ratio.