Molecular interactions are important for the function of biological macromolecular assemblies and systems. Small-angle X-ray scattering (SAXS) can provide key structural and dynamic information for biological macromolecules such as proteins, and nucleic acids and their complexes in solution, under near-native conditions. A prerequisite for accurate interpretation of solution SAXS data is that the macromolecule studied is present in monodisperse form. Only then is it possible to derive the 3-dimensional structure of proteins in high quality. In practice, many samples comprise aggregated structures and oligomeric mixtures, respectively, which complicate a precise evaluation of scattering data. To overcome this problem, SAXS can be coupled with size exclusion chromatography (SEC) and thus provides an efficient tool for screening and structural studies of biomolecular samples. SEC – which often is the final step in many protein purification protocols – separates biomolecules based on their hydrodynamic properties (size, shape). The method is specific enough to separate even different oligomeric species, such as monomers, dimers, and higher oligomers.