\ The dielectric relaxation of water-filled granular dielectrics under hydrostatic compression is investigated in the frequency domain. The activation volume and activation enthalpy governing a dominant relaxation mechanism of sandstone partially saturated with water are determined experimentally for the first time. The present results combined with recently published results for various wetted porous dielectrics indicate that, despite the complexity of this class of matter, such as heterogeneity of composition and microstructural landscape and their disordered environment, a compensation rule between the absolute value of the activation volume and enthalpy is likely to be valid. The proportionality constant is determined by the isothermal bulk modulus and its pressure derivative, according to an earlier thermodynamic elastic model. (c) 2012 Elsevier B.V. All rights reserved.