Al2O3 supports were modified by integrating USY zeolites (13.6 wt.%) of various nominal SiO2/Al2O3 molar ratios (30-80). The obtained hybrid solids calcined at 500degreesC were impregnated at incipient wetness with a Co-Mo-P solution. After sulfiding with CS2-spiked naphtha (at 230degreesC, 5.5 MPa and LHSV=3 h(-1)) the corresponding catalysts were tested in light straight-run gas oil hydrotreatment at industrially relevant operating conditions. The catalyst containing zeolite of the highest SiO2/Al2O3 molar ratio (80) showed very high hydrodesulfurization activity. Operating at 360degreesC, 5.5 MPa and LHSV=2.5 h(-1), ultra-low sulfur diesel (similar to50 ppm S) could be obtained over that material, from a feedstock of 10,400 ppm S. Medium-strong acidity could be playing an important role on this behaviour by promoting isomerization of sterically hindered organo-sulfur compounds to more favorable configurations. Moreover, hydrodearomatization and hydrodenitrogenation were also enhanced by addition of that high-silica zeolite. (C) 2004 Elsevier B.V. All rights reserved.