The effects of fluorine, phosphate and chelating agents on hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) are reviewed. All three additives enhance the activity of NiMo/Al2O3 catalysts in HDN but have only a slightly positive or even a negative effect on the HDS activity of CoMo/Al2O3 and NiMo/Al2O3 catalysts. The positive effect on HDN is due to the enhancement of the hydrogenation of aromatic rings. On the other hand, these three additives diminish the rates of C-N bond breaking and alkene hydrogenation reactions.All three additives are hard basic ligands that may interact strongly with hard acids such as coordinatively unsaturated Al3+ cations on the alumina surface. A strong interaction with the alumina support has several effects. First, molybdate and tungstate anions are no longer strongly bonded to the support and are predominantly present as polyanions, which can be easily sulfided to MoS2 and WS2 crystallites. The weaker interaction with the smaller support surface also leads to larger MoS2 and WS2 crystallites with a lower dispersion. Second, the Ni2+ and Co2+ cations will also interact more weakly with the alumina, and this makes the formation of Ni and Co promoter atoms in the catalytically active Ni-Mo-S and Co-Mo-S phases more efficient. Third, the weaker interaction of Mo and W with the support leads to a higher stacking of the MoS2 and WS2 crystallites and, thus, to the more active type II Ni-Mo-S and Co-Mo-S phases. The increased stacking is beneficial for geometrically demanding reactions such as the hydrogenation of aromatics. For less demanding reactions, such as alkene hydrogenation, aliphatic C-N bond breaking and thiophene HDS, the loss in dispersion is important. (C) 2003 Elsevier B.V. All rights reserved.