The purpose of this short paper is to illustrate an approach to the fo
rmulation of thermodynamic models for porous materials and other mixtu
res in which the constituent volume-fraction represents an independent
variable. The approach Is based on an adaptation of the MULLER-LIU fo
rmulation of the entropy inequality to such mixtures. For simplicity,
this is done for the special case of a saturated porous material in wh
ich all constituents are characterized by constant true densities, exc
hange momentum but neither mass nor energy with each other, and Behave
as isotropic, thermoelastic, viscous materials.