A series of hybrid catalysts constituted by NiMo/Al2O3 mixed with HNaY or Ni(1.45)NaY zeolite were prepared, and their activities in the hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of an untreated gasoil and a commercially prehydrotreated gasoil were compared to those of conventional NiMo/Al2O3. The influence of HNaY zeolite loading on the activity of the hybrid catalysts was investigated with both untreated and hydrotreated feeds. The addition of increasing amounts of HNaY to NiMo/Al2O3 resulted in lower activities for the HDS and HDN of the untreated gasoil, being the decrease in HDS similar for all hybrid catalysts and in HDN proportional to the HNaY content. These catalysts have a very stable activity for HDS and decaying activity for HDN of the untreated gasoil. With the prehydrotreated gasoil, a small promoter effect (similar to20%) for initial HDS activity was observed with the addition of 5 wt.% of HNaY to NiMo/Al2O3, and then a marked decrease with further HNaY loading. However, all catalysts, included NiMo/Al2O3, suffered a rapid deactivation under the reactions conditions used, which was attributed to a desulfurization of the catalysts caused by the low conversion level obtained and, therefore, a low H2S partial pressure in the reactor. The addition of Ni(1.45)Y zeolite to NiMo/Al2O3, decreased slightly the HDS and HDN of the untreated gasoil and increased by a factor of about three and nine times the initial and stationary, respectively, HDS of the prehydrotreated gasoil. Analysis of the relative distribution of sulfur species in products explains the observed synergy and the role of the Ni(1.45)NaY zeolite in the deep HDS over hybrid catalysts. (C) 2004 Elsevier B.V. All rights reserved.