Results of dielectrical measurements of properties of natural and polystyrene-modified alder wood are presented in the paper. Courses of components of complex electrical permeability of these materials within the temperature interval of 100-450 K, at frequencies of the measuring electric field amounting to 0.42 and 10 kHz were determined and compared. Changes of the structure of wood cell walls manifesting themselves in a growth of functional polar groups participating in the relaxation processes and of a partial freezing of conformation degrees of freedom of particular macromolecule fragments being carriers of these groups have been proved. The cause of structural changes can be swelling of wood matter under the effect of polystyrene penetrating composite cell walls.