The removal of sulfur and metals from a heavy residual oil by hydrotreating in a pilot trickle-bed reactor was examined. Catalysts with different active components on two different supports (bauxite and alumina) were tested. The results were interpreted via several models that took into account the physical and chemical complexity of the three-phase system. A first-order kinetics approach proved valid for the bauxite-based catalysts, having a wide pore size distribution, provided that the hydrodynamic effects in the reactor had been suitably accounted for by either external liquid holdup or apparent intraparticle dlffusivity. In contrast, the results from the alumina catalyst required interpretation with a different model, probably because of the different pore size distribution.