The angular dependence of the X-ray pure-diffraction intensity I(θ) has been measured in Ge and Si single crystals (surface covered with natural oxide films) by means of triple-crystal diffractometry. Measurements were extended to specimen-crystal angular deviations from the Bragg angle of up to 500 Bragg-peak half- widths. The I(θ) data at such large deviation angles are informative of both static and dynamic Debye-Waller-factor variation over crystal depth, the achievable spatial resolution turning out to be of the order of 1 nm. The high spatial resolution of the asymptotic Bragg diffraction made it necessary to consider in theory the layer-to-layer variation of both the scattering characteristics (Debye-Waller factor) and the interplanar spacings. A theoretical treatment of the problem is presented. Reconstructed Debye-Waller factors for the first four atomic planes, counting from the crystal-oxide boundary, are 0.3, 0.4, 0.7, 0.7 and 0.6, 0.6, 0.9, 1 for Ge and Si, respectively.