Abstract We report here the results of an investigation of thermodynamic equilibrium properties of systems constituted by Phlegraean unwelded pyroclasts, dry or saturated with water. We have experimentally measured the porosity of samples of Phlegraean rocks and determined the values of the heat capacity and of thermal expansion of dry and water-perfused tuffs having different texture structures. Also the energetics of the process of adsorption of water on the large internal surface of these porous materials has been investigated. The amount of heat exchanged during the reversible formation of an adsorbed layer has been determined, in dependence of its thickness and as a function of temperature. The coherent picture emerging from the ensemble of all these measurements is that of a direct connection between the processes of adsorption/desorption and the stress of the porous rock. In very unsaturated soils these processes may constitute a much more efficient mechanism of transformation of thermal energy into soil deformation, compared with that of thermal expansion of the solid matrix.