The experiment is aimed to unravel the diffusion mechanism of hydrogen in Callovo-Oxfordian clays, which are candidates to be used as backfill material in long term geological disposal sites for radioactive waste. Synthetic bentonites (montmorillonite) as such and pillared (with Al13-polycation) will be also studied as model systems. Both the model and the natural samples under investigations have been widely characterized for surface area, high pressure hydrogen adsorption, x-ray diffraction, thermo-gravimetric analysis and differential scanning calorimetry, and scanning electron microscopy ¿ energy dispersive spectrometry. The samples will be inserted in a sealed steel cell and hydrogen gas will be injected before starting an isochoric scan in T.