In complex porous media, the chemical potential gives the effect of the environment on one constituent (here we consider water). This thermodynamic function is rarely used in soil's mechanics. Experimental functions such as the capillary pressure, the equilibrium relative humidity ... are preferred. The chemical potential as the major advantage to be well-defined in a thermodynamic sense, like the temperature, the pressure ... For a large range of natural media (soil, gel, wood, food ...), we give the variation of the chemical potential as a function of the water content. Then we present how the chemical potential can be used to describe water transport in various situations.