A new method based on X-ray interference for the characterization of extremely thin buried single layers by means of high-resolution X-ray diffraction is presented. The independent determination of layer composition and thickness is obtained by comparing measurements performed in at least two different settings, one of which in the asymmetric grazing emergence geometry. The method is applied to study In segregation in InAs/GaAs monolayers grown by molecular beam epitaxy. In segregation lengths of nearly 6 A are measured.