Ultra thin buried InAs layers on GaAs (001) crystals prepared by molecular beam epitaxy are structurally characterized using synchrotron radiation. Grazing incidence X‐ray reflectivity and crystal truncation rods were utilized to determine the average layer thickness, interface roughness, and the stoichiometry of the layers. From X‐ray standing wave experiments the In lattice site and vertical distribution are determined. We discuss our results in view of the structural transition of the layer system with In deposition from 1.0 to 2.1 ML.