In industrial practice, hydrotreating of oil fractions is carried out in either a gas-phase process or a trickle flow process. We previously noticed that a remarkable difference exists between the relative activity of mixed sulfide catalysts in gas-phase and liquid-phase hydrodesulfurization (HDS) reactions. In the literature, however, no satisfying explanation with respect to the possible fundamental differences between these reactions can be found. In this paper, we report an elaborate investigation on the effect of reaction conditions, type of reactant and type of the catalyst on the occurrence of differences between the relative activity, i.e. ranking, of mixed sulfide catalysts in gas-phase and liquid-phase reactions. Striking differences were observed between the ranking of nitrilo-triacetic acid (NTA) and conventionally prepared NiMo catalysts in thiophene gas-phase HDS and liquid-phase dibenzothiophene (DBT) HDS. Importantly, these differences did not depend on the nature of the reacting sulfur-containing compound. This allows the generalisation that NTA-based Ni(Mo) catalysts are relatively more active in gas-phase HDS reactions, whereas conventionally prepared NiMo catalysts are relatively more active in liquid-phase HDS reactions. An analogous behaviour was observed for low- and high-temperature sulfided NiW/gamma -Al2O3 catalysts, of which the latter is much more active in gas-phase HDS reactions and the former is more active in liquid-phase HDS reactions. It is concluded that this so-called 'gas-liquid-phase controversy' is a generic phenomenon in hydrotreating reactions over metal sulfide catalysts. It was verified that mass transfer limitations do not play a role in this matter. The active sites of stacked slabs of the type II catalysts are more affected than those of type I catalysts, in which the active phase is in a more close interaction with the support. It is proposed that the phenomenon is related to a non-selective competitive adsorption of the a-polar solvent molecules on sites protruding from the catalyst surface. Apparently, the proximity of the ionic surface of the alumina support hinders the adsorption of the a-polar hydrocarbon molecules on the non-stacked systems, whereas the sulfur- and nitrogen-containing molecules are not so much affected in their adsorption behaviour on these active sites.