Résumé : The different methods used for the determination of the monolayer capacity from isotherm adsorption measurements have been applied to the H2 adsorption results on the EUROPt-1 reference catalyst. These are analysed to show how some of these methods are dependent upon experimental features such as the evacuation rate at the catalyst surface, the size of the catalyst sample. It is shown that isochore measurements, in contrast to isothermal measurements, allow determination of variations in the isosteric heat of adsorption Qiso over a large coverage range, and up to the monolayer saturation. The variations of Qiso with the adsorbed amount allows us to propose a new way of determining the value of the monolayer capacity. The values determined by isothermal measurements are discussed at the light of this new method. © 1993.