The different catalytic sites involved in hydrodenitrogenation (HDN) over supported sulfided Ni-Mo and Rh catalysts were investigated by means of four model reactions, the hydrodenitrogenation of methylcyclohexylamine (MCHA) and o-toluidine and the hydrogenation of cyclohexene (CHE) and toluene (TOL). By comparing the effects of the silica-alumina support and fluorination on the activity of these reactions, at least four sites were identified. The active site for C(sp(3))-N bond cleavage involves neighbouring acid and base sites. The site for C(sp(2))-N bond cleavage is highly unsaturated and accommodates aromatic amines in a flat adsorption mode. Hydrogenation of aromatics is favoured by high stacking of the MoS2 slabs with more edge and corner sites, whereas the hydrogenation of alkenes is favoured on catalysts with better dispersion and lower stacking Of MoS2 slabs. (C) 2003 Elsevier Science B.V. All rights reserved.