As follows from the review of recent publications information about the structural interaction membrane-penetrant is so far fragmentary. The capabilities of X-ray methods in this field have been described for the system : cellulose-water (light and heavy). Two methods were applied : WAX and SAXS. The following information has been obtained : WAX - existence of structural changes, different for H2O and D2O, at intermolecular level after water-vapor treatment of cellulose, and existence of bulk water after soaking cellulose into water. In order to obtain better presentation the structural changes, a method of measurement and calculation of interference function, F(k), in a limited range of k, has been elaborated. SAXS - existence of water clusters in cellulose soaked in water and calculation of their approximate sizes.