A series of NiMo catalysts supported on Al2O3-ZrO2 composites containing 0, 2.5, 5 and 10 wt.% ZrO2 was synthesized, characterized and evaluated for deep desulfurization of gas oil. X-ray diffraction patterns of the supports indicate homogenous dispersion of 2.5-10 wt.% of ZrO2 in bulk Al2O3. Mono-modal pore-size distribution was observed with a decrease in average pore-size with increasing ZrO2 content. NH3TPD results show that incorporation of 5 wt.% or more ZrO2 neutralized the weak acid sites of Al2O3 and generated a different type of stronger acid sites. UV-Visible diffuse reflectance spectroscopy results indicate the presence of tetrahedral and octahedral Mo6+ ions species along with an increase in octahedral species with addition of ZrO2 possibly due to weaker interaction of active metals and composite support. This result is also supported by H-2-TPR and CO chemisorption measurements. Deep desulfurization of gas oil, carried out in a bench-scale flow reactor at 320, 340 and 360 degrees C, indicate that the addition of ZrO2 increased the catalytic activity - especially at higher temperature. Compared to Al2O3-based catalyst, the 1.5 order HDS rate constant was about 1.3, 1.8 and 2.5 times higher for catalysts containing 2.5, 5 and 10 wt.% ZrO2, respectively. A correlation was found between the enhancement of hydrogenation activity of sulfided catalysts and the reducibility of their oxide precursors, as determined by the amount of hydrogen consumed in TPR experiments followed by mass spectroscopy. (C) 2012 Elsevier Ltd. All rights reserved.