The effect of removal extent of nitrogen-containing species on the hydrodesulfurization (HDS) of straight-run gas oil from a Mexican refinery over a NiMo-based catalyst was studied. The original middle distillate (similar to 360 ppm N) was subjected to N-compounds removal by contacting with a proprietary adsorbent. By varying contact time, feedstocks of similar to 234 and similar to 137 ppm N were obtained. Progressive improvement on HDS over NiMo catalyst was observed by decreasing concentration of organo-N compounds in the gas oils tested in batch reactor. However, this behavior seemed to depend on the severity of the operation. By increasing operating temperature, the enhanced desorption rate of N-compounds relieved their poisoning effect, making less pronounced the benefit of low-nitrogen feedstocks. The HDS activity data from batch reactor were compared to those found when equivalent experiments were carried out in a fixed-bed up-flow reactor under conditions similar to those used in industrial facilities. (C) 2006 Elsevier Ltd. All rights reserved.