Alumina-silica-supported NiMoS composites were examined in single- and dual-layer catalyst beds in a high-pressure (5 MPa) flow reactor to achieve ultra low sulfur (10 ppm) diesel fuels. Three types of alumina-silica composite supports were prepared by co-precipitation to control the particle size and arrangement of alumina and silica. The SiO2 content was found to be influential on catalytic performance, being best by around 27% regardless of preparation conditions. Alumina crystal size controlled the acidity and surface area of the support, key factors influencing catalytic performance. NiMoASA-2(27). prepared by procedure 2, achieved 4.5 and 3 ppm S at 345 and 360 degrees C, respectively, in the single bed reactor at a liquid hourly space velocity (LHSV) of h(-1). NiMoASA-2(27) achieved the best performance of the supports examined in this study. The double-layer catalyst bed contained commercial CoMoS (LX6) and NiMoASA-2(27) in the first and the second beds at 345 and 360 degrees C. respectively, and achieved 5 and 2 ppm S. indicating better performance at higher temperatures. The reaction order for the hydrodesulfurization (HOS) of refractory sulfur species was close to unity over NiMoASA-2(27). which was significantly higher than that of the commercial CoMoS catalyst. Alumina-silica supported NiMoS in the second bed of the dual-layer catalyst bed achieved less than 10 ppm S for refractory sulfur species with around 500 ppm S. (c) 2011 Elsevier B.V. All rights reserved.