Cell walls of so-called “wood-materials” are constituted by a complex, highly anisotropic and hierarchically organized nanocomposite, characterized by stiff crystalline cellulose nano-fibers, parallel to each other, and embedded in a softer and less anisotropic matrix of hemicelluloses, lignin and water. This matrix is hygroscopic, and therefore swells with increasing humidity. Consequently, wood cells undergo large dimensional changes. A minimal model of wood secondary cell walls to predict water absorption has recently been developed by Bertinetti and co-workers [1] in the form of an Equation of State (EOS) that represents equivalently the water sorption versus relative humidity, as considered in chemical engineering, or the relation between osmotic pressure and volume of solutes, in the physical chemistry equation of state approach initiated by Jean Perrin. We extend hereby this model to the presence of electrolytes adsorbed in the gel part wood cell wall and compare compression wood cell walls to the extreme case of coir.