Mechanical mixtures of macromolecules and simple solutes were investigated with respect to their water adsorption capabilities. The additivity principle was applied to predict sorption isotherms of mixtures on the basis of the sorption behaviour of pure components. Mechanical mixtures of macromolecules and mixtures of biopolymers and simple solutes yielded isotherms which were superimposable with those predicted at the low water activity range. III the multilayer water absorption range, predicted isotherms in most cases overestimated the water contents of mixtures in comparison with experimental values. The reasons for the observed differences between experimental and predicted values are discussed. These may include the ordering of tile water in contact with macromolecule surfaces, swelling, conformational changes, polymer-polymer interactions multiple hydrogen bonds, binding of ions, crosslinking, competition for water and plasticization of amorphous regions. The rates of these changes are limited and long rime-scale experiments may be needed to observe equilibrium sorption isotherms of mechanical multicomponent mixtures. It is suggested that lack of tile additivity of sorption processes should be rather a rule, not an exception in the case of multicomponent mixtures.