The interaction of water with porous media is still a controversial topic. Hydration of most materials softens them but numerous exceptions exist. The visco-elastic properties are also affected and two experimental facts are of interest. Firstly, the damping factor increases with the coverage up to a monolayer. Secondly, it presents a maximum beyond this coverage. During drying, this huge peak has been interpreted as a mechanosorptive effect, i.e. stress assisted adsorption. During adsorption, the stress state of H-bonded solids changes, leading to enhanced viscoelastic dissipation. This first paperprovides experimental evidences of the H-bonding in solids, a detaileddiscussion of the mechanisms operative in mechanosorption is postponed to a forthcoming one.