The hydrodesulfurization (HDS) of five straight run gas oil (SRGO) samples with different nitrogen concentration (0.0020-0.0357 wt.%) and sulfur concentration (1.3985-1.2130 wt.%) was carried out in a pilot plant under the following conditions: LHSV = 2.5 h(-1), Pressure = 5.49 MPa, Temperature = 623, 633 and 643 K with a commercial CoMoP/gamma-Al2O3 catalyst. There was an important inhibition effect even with trace amounts of nitrogen compounds. The data was analyzed using a rate model which includes the nitrogen inhibition effect. The HDS rate varied proportionally to the 1.5 power of sulfur compound concentration and inversely with the nitrogen compound concentration in the SRGO. The nitrogen compound adsorption parameters and HDS rate parameters for SRGO were calculated. The results show that it is possible to obtain ultra low sulfur diesel (ULSD) by removing the nitrogen compounds before the HDS process. (C) 2014 Elsevier Ltd. All rights reserved.