Deep hydrodesulfurization (HDS) was investigated over amorphous silica-alumina (ASA), produced using various synthetic methods as supports for NiMo catalysts, in single and dual catalytic bed systems. NiMoS active components supported on ASA-2 (ASA from synthetic method 2) achieved S levels of 4.5 and 3.0ppm at 345 and 360 degrees C, respectively, in single catalytic beds, which was the highest HDS reactivity. Dual catalytic beds were introduced to inhibit excess hydro-cracking of C species in gas oil. For the dual catalytic combination of LX6 (commercial catalyst for reactive S species) and NMASA-2 (NiMoS supported on ASA-2), the S levels were 5.4 and 2 ppm at 345 and 360 degrees C, respectively, indicating that the performance was enhanced at higher temperatures, likely due to improved activity for refractory S species. The increased reactivity for HDS over NMASA-2 was related to the strong acidity of this support due to tetrahedrally coordinated Al species. In addition, the smaller homogeneous particle size of the support influenced the HDS catalytic performance. Moreover, weaker interactions between the Mo and ASA-2 in NMASA-2 improved the HDS reactivity, as confirmed by laser Raman spectroscopy. (C) 2010 Elsevier B. V. All rights reserved.