An understanding of the behaviour of saturated/unsaturated soils is important for the design and analysis of geostructures. In this paper a continuum approach based on a structured thermodynamic theory of mixtures developed by Svendsen & Hutter (1995), in which the constituent volume densities play the role of internal variables, is applied to the formulation of field equations for saturated/unsaturated compressible/incompressible soils. Emphasis is on the presentation of differences in the formulation and the roles of thermodynamics. It is seen that this approach does specialize to conventional mass and momentum balances as well as constitutive relations in saturated and unsaturated deformable porous media. With this result it is shown how some of well-known coupled theoretical formulations describing the deformation of soils are thermodynamically consistent. For example, we establish a thermodynamic basis for effective stress in saturated and unsaturated soils and delineate the applicability of Terzaghi’s effective stress concept.