Mechano-sorptive strain was generated in strips of rectangular section stressed in torsion with the humidity cycled between 38% and 84%. After a number of humidity cycles the specimen was unloaded, the applied stress was reversed and further humidity cycles were imposed. All the initial strain was recovered and reversed strain was developed. Tests were carried out at two maximum shear stress levels, 1.12 MPa and 5.05 MPa. After seven humidity cycles at the higher stress a large mechano-sorptive strain of about 0.06 was developed, which was nearly nine times the initial elastic strain. During the development of this strain no significant change of the shear modulus occurred, from which it is concluded that the large MS strain produces no permanent change of the features of the cell wall structure that determine the shear modulus. There are close parallels between the mechanism generating the MS strains and that responsible for the large plastic strains reported by Keckes et al. in wet compression wood loaded in tension. In both cases it is suggested that the shear strains arise from the breaking and reforming of hydrogen bonds between parallel polymer chains.