Two types of alumina-USY supported NiMo catalysts (NMAZ and NMACZ) were examined in an autoclave for HDS of straight run gas oil (SRGO; S = 11780 ppm) and its hydrodesulfurized gas oils (HSRGO; S = 340 ppm) to simulate a first and second layer catalyst. One of the supports was a physical mixture of alumina and USY type zeolite (AZ), while the other was surface modified USY type zeolite with alumina (ACZ). Their supports alone (AZ and ACZ) were examined also to clarify the function of zeolite support during hydrodesulfurization (HDS) of gas oil. NMAZ was found to be active enough for HDS of SRGO to reduce sulfur content to 242 ppm. The extensive cracking to produce light hydrocarbons is another characteristic of that catalyst. NMACZ was inferior to NMAZ in HDS of SRGO but superior in HDS of HSRGO in the presence of H2S and NH3, achieving 10.3 ppm of sulfur. No cracking was observed over NMACZ which is another advantage of this catalyst. Extensive HDN took place over NMAZ but only moderate HDN over NMACZ. NH3 TPD and Raman spectroscopy of oxide forms of the catalysts indicate lower acidity of both catalysts and slightly better dispersion Of MoO3 over the NMACZ, which had lower surface area than NMAZ. Zeoite-containing support alone adsorbed and desulfurized refractory sulfur species, especially 4,6-dimethyidibenzothiophene contained in HSRGO. HDS is suggested to be favored over the zeolite-containing supports by such mechanisms, in addition to their surface acidity, to enhance the hydrogenation and resistivity against H2S inhibitions. (C) 2004 Elsevier B.V. All rights reserved.