Gas oils from heavy crude oils tend to have higher sulfur, nitrogen, and aromatic contents. Hence, better processes and catalysts need to be developed based on the exact chemistry and chemical reactions of the feedstock. Saudi Arabian gas oils obtained from Arabian light crude oil (AL-GO), medium crude, and heavy crude (AH-GO) were characterized and their HDS reactivities were studied. AH-GO showed obviously more sulfur, refractory sulfur, nitrogen, and aromatics. Hence, its difficulty of deep HDS in the single stage was proven while AL-GO was easier to achieve 10 ppm sulfur in product within 2 hr. Two-stage HDS was very effective in achieving 10 ppm sulfur in product with AH-GO. Most of the catalysts achieved < 10 ppm sulfur in product at 360°C. Acidic NiMo catalyst appeared effective for deep HDS although the highest activity was obtained with CoMo/Al2O3 particularly designed for ultra-deep HDS. This is an abstract of a paper presented at the 20th Annual Saudi-Japan Symposium on Catalysts in Petroleum Refining and Petrochemicals (Dharan, Saudi Arabia 12/5-6/2010).