Two NiMo sulfide catalysts, supported on Al2O3 and silica-alumina, respectively, were compared in the hydrotreatment of a mixture of straight-run and coker gas oil. Experiments were conducted in a continuous fixed bed reactor, at 623-643 K and 5 MPa. The Al2O3-supported catalyst proved to be slightly more active in HDN and significantly more active in HDS than its silica-alumina analogue. The activity trend was attributed to the inhibition provoked by the strong adsorption of basic nitrogen species. Earlier work had shown that the adsorption (and thus the inhibition) was significantly stronger on an acidic support. Our data further showed that the reactivity of neutral (pyrrole benzologues) and basic nitrogen compounds was different. The basic fraction was converted in a gradual fashion, whereas the large majority of neutral species was converted very quickly, except for a small fraction, which remained practically inert to hydrotreatment.