The sulphidation of a CoMo/Al2O3 hydrotreating catalyst was studied under pressure and flow conditions close to industrial practice. The sulphiding mixture contained equal amounts of H2S and CH4 diluted in hydrogen under 3.9 MPa total pressure. The oxidic precursor was flushed by the reagent gas at 40 degreesC for 10 h, then the temperature was raised to 400 degreesC within 2 h. Mass variation was continuously recorded by means of a suspension magnetic balance, and the gas phase was analysed by mass spectrometry. Sulphur contents were also determined at intermediate temperatures by chemical analyses. Thus, mass variations due to the presence of adsorbed species or to the formation of CoMo sulphides could be distinguished. The rapid gain in mass noted at 40 degreesC was mostly due to adsorption of H2S over the oxidic solid, since it increased with H2S partial pressure. At this stage, however, a small amount of the reactive oxygen was already exchanged for sulphur. Upon raising the temperature, the mass decreased due to a combination of desorption and sulphiding steps. Above 140 degreesC, H2S consumption was evidenced, together with water production, and the rate of sulphiding increased with the H2S partial pressure. At 300 degreesC, the mass variation was close to that expected for complete transformation into Co9S8 and MoS2. Above 350 degreesC, the mass further increased due to the replacement of adsorbed water by H2S. At 400 degreesC, an important excess mass was observed at all H2S partial pressures. Thus, the adsorption sites on the metal sulphides are essentially saturated by H2S species under practical conditions.