The paper discusses the physical relations between wood at a plane state of stress and variable humidity. A simple theory for wetted wood under stress is constructed based on the balance equations and the thermodynamics of irreversible processes. The theoretical curves are compared with experimental curves for pine sapwood wetted at uniaxial and biaxial states of stress. The so-called mechano-sorptive effect, which arises during the simultaneous action of mechanical loading and wetting process, is taken into account. A high consistency of the theoretical and experimental curves is stated.