Sulfidation of trimetallic CoNiMo/Al2O3 catalysts was studied by thermogravimetry at 400 degrees C under flow and pressure conditions. Results were compared with those obtained on prepared and industrial CoMo/Al2O3 and NiMo/Al2O3 catalysts. The amount of sorbed H2S on the sulfided solids was measured at 300 degrees C in the H2S Pressure range 0-3.5 MPa at constant H-2 pressure (3.8 MPa). The adsorption isotherms were simulated using a model featuring dissociated adsorption of H2S on supported metal sulfides and bare alumina. The amount of sulfur-vacancy sites could thus be determined under conditions close to industrial practice. A relationship with activity results for thiophene HDS and benzene hydrogenation was sought for. (c) 2005 Elsevier B.V. All rights reserved.