The sulfiding behaviour and hydrotreating performance of CoMo and NiMo on carbon catalysts were investigated, with a view to establishing the suitability of an active-carbon carrier for industrial hydrotreatment purposes. The sulfidation process (H-2/H2S at 0.1 and 4 MPa) was followed with Co-57 Mossbauer emission spectroscopy, and X-ray absorption spectroscopy (Mo- and Co-edge). It turns out that the structural evolution of the CoMo/C catalyst is in broad terms very similar to that of a CoMo/Al2O3 one, including the type I -> II phase transition at increased sulfiding pressure, albeit that some sintering takes place concomitantly. In the hydrotreating part a NiMo/C catalyst was employed, sulfided at I or 4 MPa, in trickle-flow conditions at 3.5 (HDS) and 6 (HDN) MPa. It transpires that the adsorption properties of NiMo/C are quite different from those of a NiMo/alumina, in that dibenzthiophene, quinoline, and polyaromatics are much stronger adsorbed on the carbon-supported catalyst. This leads to very high dibenzothiophene HDS and quinoline HDN activities, but to a disappointing performance in the hydrotreatment of a heavy gasoil, where the polyaromatics can compete effectively for adsorption on the active HDS/N sites. (c) 2013 Elsevier B.V. All rights reserved.