A synchrotron x-ray diffraction method is presented for structural investigations of interfaces between low-Z substrates and heavier liquids. The method, similar to methods used in neutron scattering, is based on illuminating the interface through the solid substrate. The backgrounds arising from bulk scattering and the signal-to-background ratio are estimated and compared with experimental results. An ultrahigh vacuum (UHV) setup is described in which the atomic arrangement and roughness of clean interfaces can be studied in situ. Our first results illustrate the possibilities for both out-of-plane and in-plane diffraction studies. The specular reflectivity of the Ga/diamond(111)-2×1 interface was measured for perpendicular momentum transfers up to 2.2 Å−1. In an in-plane study of Ga/Si(111)-7×7 the in-plane structure factor of Ga liquid within a depth of ∼ 50 Å was compared to the structure factor of the bulk liquid. © 1997 American Institute of Physics.