X-ray reflectivity using high-energy X-rays allows one to characterize interfaces between thick materials at nanometer scale. The technique combines the high penetration of X-rays allowing the crossing of the radiation through large thicknesses of material with the interface sensitivity of grazing angle techniques. In the case of a buried interface between two thick materials, the beam enters the sample through the side of one material and contributions of external surfaces are suppressed. Then, the technique is sensitive to the interface structure only. Examples are given using wafer bonding interfaces, both in the hydrophilic case (as used e.g. in Silicon-On-Insulator substrate fabrication) and in the hydrophobic case (Silicon/Silicon bonding).