A comprehensive investigation aimed at elucidating thermodynamic and mechanical behavior of heterogeneous systems,,featuring an association of solid hydrophobic matrices (micro- or mesoporous) and water, has been undertaken. Scanning transitiometry techniques were applied for measurements of mechanical work and heat effects during cycles of isothermal compression/decompression. The interface variation during forced intrusion of water into the matrix porous space and its spontaneous expulsion show complicated thermal behavior patterns. Typical exothermic intrusion and expulsion are confirmed for certain, categories of systems, whereas for others strong endothermic intrusion is detected. A simple thermodynamic model involving variable irreversibility in the liquid flow and in the contact line movement is proposed to explain such an original thermomechanical behavior. Consequences for mechanical devices based on these heterogeneous systems are fully discussed. (c) 2005 American Institute of Chemical Engineers