The amount of H2S adsorbed on a sulfided CoMo/Al2O3 hydrotreating catalyst has been measured under flow conditions using a high pressure thermogravimetric technique. The measurements have been performed under a wide range of conditions: at 313-573 K with a mixture of H2S and CH4 (0.015-0.35 MPa) obtained from the decomposition of dimethyldisulfide diluted in H-2 (1.8-3.8 MPa). Five isotherms determined at temperatures below, near, and above the critical temperature of H2S, were established from isobaric measurements. The adsorbed phase was found to consist essentially of H2S. The thermodynamics of H2S sorption was studied in order to obtain information about the state of the adsorbed phase and the mutual interaction between adsorbed H2S molecules or dissociated H2S species, and about the homogeneity or the heterogeneity of the catalyst surface. The experimental isotherms were compared to 15 isotherm models featuring mobile adsorption and localized molecular or dissociative adsorption on a single-, two- and multi-site surface. Discrimination among rival models was based on statistics and theory. Two models were retained: the generalized Freundlich model featuring adsorption of H2S molecules on a patch-wise distribution of sites, and the Langmuir model characterized by the dissociative chemisorption of H2S on dual-sites on both CoMo sulfide phases and alumina. Using the latter model, the amount of adsorbed H2S on the supported CoMo sulfide phases was determined under conditions close to industrial practice.