Membranes of archaebacteria show some very interesting features, the most prominent being their stability at high temperatures. This property seems to be caused by the bipolar nature of the membrane lipids. The configuration of the lipid molecules in a monolayer of a native mixture of lipids extracted from Sulfolobus solfataricus has been studied by X-ray reflectivity of samples on solid substrates. Films on substrate were prepared by Langmuir-Blodgett techniques. We used both pure silicon (hydrophilic) and silicon covered by dimethyldichlorosilane, rendering the surface hydrophobic. Data were analysed using a box-model and the recursion scheme of Parratt. The results for the samples on hydrophobic substrate are interpreted by a stretched horizontal configuration. The electron densities and thickness of the layers are in good agreement with a model of close-packed molecules lying flat on the surface. Samples on pure silicon show a U-shape form of the molecules.