The structure of MCM-48 mesoporous silicate materials has been fully characterized from X-ray diffraction data by applying recently developed methods of mesostructure analysis and full-profile refinement. The pore wall thickness of both as-made and calcined MCM-48 was determined with high precision to be 8.0(1) Angstrom. No regular variations of the wall thickness were detected, but its density was found to be ca. 10% higher in the low-curvature regions. The surfactant density in the pores was assessed around 0.6-0.7 g/cm(3) and was found to have a distinct minimum in the pore center similar to that detected previously in MCM-41. A new extended model function of the density distribution in MCM-48 is proposed on the basis of the structural features that were revealed.