An improved process for high-quality diesel fuel production by hydrotreating atmospheric gas oil (SRGO) and light cycle oil (LCO) blends is presented in this paper. For this purpose. a set of blends of 5, 10 and 15% by volume of LCO with final boiling points of 300. 325 and 350 degreesC with a full range Gas oil (FBP 350 degreesC) was hydrotreated in a pilot plant at 340-380 degreesC, 5.4 MPa, 2.5 h(-1) LHSV using a commercial Co-Mo catalyst. A relationship between the concentration of refractory sulfur compounds (those boiling above 316 degreesC) and araromatics content in the feedstock with the hydrotreating temperature required for meeting a 0.05% sulfur specification was found. The experimental data obtained during the desulfurization was quantitatively represented by a 1.50 to 1.56 order rate equation. with activation energies between 18.9 and 34.1 kcal/mol, depending on the feedstock. (C) 2004 Elsevier Ltd. All rights reserved.