To study the effect of reactor configuration on hydrotreating, a series of experiments was carried out in a pilot plant equipped with two reactors in series that can operate in two modes: a fixed-bed reactor (FBR) or an ebullated-bed reactor (EBR). The experiments were carried out with the atmospheric residue as feedstock and commercial catalysts at a pressure of 100 kg/cm(2) and temperatures of 380 and 400 degrees C. The highest conversions were obtained for the EBR-EBR (35.67-52.21%) and EBR-FBR (29.08-53.09%) configurations, unlike the FBR-FBR configuration that exhibited the lowest conversion (15.42-27.34%). At 400 degrees C, the EBR-FBR and FBR-FBR configurations showed higher hydrodemetalization than the others (81.00 and 76.95%, respectively). It was confirmed that the formation of sediments increases at high temperatures, whereas at 400 degrees C, it begins to be significant for the EBR-FBR and EBR-EBR configurations (0.010-0.767 and 0.041-0.613 wt %, respectively), in contrast to the FBR-FBR configuration that presented lesser formation of sediments (0.003-0.009 wt %).