The evaluation and ranking of hydrotreatment catalysts at laboratory scale is generally performed with model compounds, thiophene or dibenzothiophene (DBT), either in the gaseous or in the liquid-phase. According to the considered reactant and the chosen conditions, confusing data have been published. The presence of an inert solvent has been claimed to affect the measured activity but no comprehensive experimentation supports this assertion. Some of the possible causes for the observed discrepancies include the inhibiting effect of hydrogen sulfide, the distribution of the reactants in the gaseous and liquid and the control of transport phenomena.The rate of tetralin hydrogen was determined in a micro up-flow three-phase reactor, the reactant being either in the gaseous or the liquid-phase, as a pure compound or in the presence of inert solvents. Expressing the reaction rate as a simplified form of the Langmuir-Hinshelwood formalism, the reaction rate constant and the relative tetralin-to-solvent adsorption constant ratio were determined. The rate constant is independent of the reaction conditions (gas- or liquid-phase) and of the nature of the solvent, indicating that the intrinsic activity of the active phase does not depend on the experimental conditions. The values of the relative adsorption constant indicate that the competitive effect is more pronounced as the molecular weight of the solvent increases. (C) 2003 Elsevier B.V. All rights reserved.