The effects of the removal extents of nitrogen containing species in gas oil on its deep hydrodesulfurization (HDS) was studied by using conventional catalyst and reaction conditions. The 80% denitrogenated gas oil achieved the total sulfur content of 19 ppmS by hydrotreating at 340degreesC while the feed and 64% denitrogenated gas oils resulted in 311 and 120 ppmS of total sulfur contents, respectively under the same conditions. Selective removal of refractory sulfur species which occurred simultaneously with nitrogen removal over a proprietary adsorbent was recognized very effective to achieve deep HDS. (C) 2003 Elsevier B.V. All rights reserved.