Many polymeric substrates which swell on adsorption exhibit sorption hysteresis; the phenomenon is most striking for water sorption by natural polymers and proteins. Some interpretations of hysteresis for swelling systems have invoked the concept that there are more active sites available for association with sorbate during desorption than for adsorption to the same with relative pressure. Chemical modification of hydrophilic groups, which markedly alters the amount of water sorbed by keratin, or the filling of possible voids in the substrate has little effect on keratin-water vapor sorption hysteresis. Sorption in swelling systems occurs by a coupled diffusion-relaxation mechanism. It is demonstrated that the occurrence of hysteresis is associated with the stress relaxation of the cohesive forces opposing swelling. Changes in the structural conformation may be considered as the variable which differs between the adsorption and desorption states. The segmental mobility of the macromolecular chains plays a major role in the irreversibility of the sorption isotherm, leading to wide diversity in hysteresis effects in polymers.