The experimental isosteric heat, Q(st), for molecular hydrogen adsorption in a Cu-exchanged zeolite ZSM-5 was determined as a function of the hydrogen loading. Q(st) was found to vary in a stepwise manner between 73 +/- 5 and 39 +/- 4 kJ/mol H-2. This behaviour suggests the presence of different groups of adsorption sites, some of which have the appropriate thermodynamics necessary for an ambient conditions sorption based H-2 storage medium. Our data can be directly related to model calculations and inelastic neutron scattering spectra. From a thermodynamic point of view, they confirm the feasibility of hydrogen storage applications based on molecular chemisorption of hydrogen. (c) 2007 Elsevier B.V. All rights reserved.