X-ray interferometry has been used to btudy the profile structures of chemisorbed alkylsiloxane (hexadecyl-, octadecyl-, and eicosyl-) monolayer films on Ge/Si multilayer substrates of the type 2(Ge&o), Le., two superlattice unit cells, each consisting of 2 Ge monolayers and 30 Si monolayers, as fabricated by molecular beam epitaxy (MBE). Analysis was performed in three steps. First, based on the structural specifications of the cleaned, bare substrates acquired from the MBE fabrication, the actual profile structures of selected multilayer substrates were determined via a model refinement analysis of their meridional X-ray diffraction. The partial profile structures of these substrates (e+, GezSi3oGez) were then employed as the reference profile structure to perform a highly constrained, real-space refinement analysis to derive the profile structures of these multilayer substrates including the particular self-assembled monolayers on their surface, as relative electron density profiles from their meridional X-ray diffraction. Lastly, a real-space refinement was applied to convert the relative electron density profiles to their equivalent profile structures in absolute electron density. These profile structures demonstrate the alkyl chain configurations in these self-assembled monolayers are all-trans and tilted, with respect to the surface normal. The tilt angles are approximately 32 f 3O, 31 * 3O, and 30 * 3' for the 16C, 18C, and 20C chains, respectively. The terminal methyl endgroups of each monolayer seem to be well ordered in the profile structure, all lying in the same surface plane to within <*l A. We also discovered that a layer of SiO, between 15 and 18 A thick was formed at the surface of the substrates during our prealkylation cleaning procedures. The profile structures so-derived via X-ray interferometry were further proven to be correct by inspection of the Patterson function, derived uniquely from the meridional diffraction without phase information, by analogy to off-axis Fourier holography. This overall approach, which does not require either the determination of explicit phase information or the preassumption of any model parameters, is therefore very powerful for uniquely determining the unknown profile structure of the organic overlayer including the substrate/overlayer interface. The in-plane structures of the 16C and 20C self-assembled monolayers were also investigated by high-resolution X-ray diffraction. In both cases, the in-plane structure was found to be distorted hexagonal (b/a = 2.08, area/chain = 17-19 A2, correlation length =: 260 A) with a nearest-neighbor chain tilt direction.