In this paper, a comparative study of the influence of chelating agents on the hydrodesulfurization (HDS) activity of gamma-Al2O3- and SiO2-Al2O3-supported CoMo catalysts is reported. A series of CoMo catalysts containing conventional and high metal (Mo and Co) loadings were prepared by coimpregnation using SiO2-Al2O3 and gamma-Al2O3 supports with and without adding complexing agents [e.g., ethylenediaminetetraacetic acid (EDTA)] to the impregnating salt solutions. The HDS activities of all prepared catalysts were tested in a microreactor using straight run gas oil as feed. The addition of a complexing agent [EDTA or nitriloacetic acid (NTA)] to the impregnation solution resulted in a remarkable improvement in the HDS activity of both gamma-Al2O3- and SiO2-Al2O3- supported CoMo catalysts. Catalysts prepared with complexing agents (EDTA or NTA) were more active in the uncalcined form, and a substantial reduction in the HDS activity occurs upon calcination of the catalyst. Catalysts prepared with SiO2-Al2O3 support were remarkably more active than those prepared with gamma-Al2O3 support. When higher metals loading together are combined with the use of the SiO2-Al2O3 support (instead of gamma-Al2O3) and an improved impregnation technique involving the addition of a complexing agent such as EDTA, highly active HDS catalysts can be prepared for deep desulfurization of diesel to ultralow sulfur levels (< 15 ppm).