Conventional analytical techniques such as the Langmuir-Blodgett method and grazing-incidence small-angle X-ray scattering (SAXS) have advanced our understanding of these assemblies. To initiate self-assembly, controlled volumes of nanoparticle solution were added to the liquid surface, and solvent evaporation drove the assembly. Advantages of the Vertical SAXS ApproachThe vertical SAXS geometry provided several advantages over traditional grazing-incidence techniques. Implications For NanoscienceThis research demonstrates that combining vertical SAXS with optical microscopy provides a powerful, in situ method for studying nanoparticle self-assembly at liquid interfaces. Monitoring nanoparticle self-assembly on liquid subphases in situ in a vertical scattering geometry.