Detailed characterization of extremely thin buried Ge films of monolayer thickness (δ layers) was performed by combination of grazing incidence X-ray reflectivity, crystal truncation rods, and X-ray standing waves. Grazing incidence reflectivity and crystal truncation rods are used to determine the average layer thicknesses and interface roughnesses as well as the stoichiometry of the layers. X-ray standing wave measurements give an independent measure of the epitaxial quality of the Ge film. Ge δ layers on Si(001) were grown by molecular beam epitaxy (MBE) at 350°C with coverage ranging from 1 to 6 ML. We find strong segregation of the first deposited Ge monolayer with subsequent Si deposition. Upon deposition of more Ge, additional Ge forms a Ge film at the Si/Ge interface. The thickness of this film is limited by Ge island formation for Ge coverage exceeding 3–4 ML.