Abstract: An asphaltene-containing oil hydrodesulfurization process employing a guard reactor and a main reactor in series, with the diameter of the guard reactor being smaller than the diameter of the main reactor. The catalyst in the guard reactor comprises particles of supported Group VI and Group VIII metals in the form of elongated extrudates whose surface is provided with a plurality of alternating longitudinal grooves and protrusions. An advantageous mass velocity-related activity effect is demonstrated when catalyst particles having this shape are disposed in a guard reactor having a smaller diameter than the diameter of the main reactor with which it is associated.