The formalism of generalized thermodynamics is applied to moist soil, as a three phase system, to derive expressions for the thermodynamic functions of each phase in terms of well defined characteristic variables. While there is no special problem in assigning proper expressions to the thermodynamic functions of the solid and gas phases, the water phase must be given special treatment due to the binding of water by the solid matrix. A new set of state variables is introduced which allow the thermodynamic description of soil water as a separate thermodynamic system, independent of its matric environment. The soil water matric potential is shown to play a central role in the expression of the fundamental thermodynamic relationships. Although the relations derived apply only to nonswelling soils with pure water as the liquid phase, the analysis can be readily generalized to include swelling and the presence of solutes. The present treatment can also be applied to porous systems other than soil or liquid phases other than water.