The paper examines the advantages of the SAXS method in the investigation of the swelling degree of the polymer layer formed on rigid porous materials. Among others, the SAXS allows for the estimation of the value of interfacial surface as well as the value of the volume fraction of the material, which is related to scattering objects. The swelling process leads to change in the radius of pores and, in consequence, change in the volume fraction of solid phase. The paper presents the theory which allows for calculating the influence of this change in increment of polymer thickness on SAXS data as well as the experimental results of the SAXS study of the porous materials composed of controlled porosity glass with formed polysaccharide-polyimine surface layers differing in cross-linking degree. The presented results reveal the usefulness of the SAXS method in investigating the swelling degree of polymer layers formed on the surface of rigid porous materials. This is because obtained values from the SAXS data are in agreement with the knowledge of the physicochemical properties of polymers.