Analytical ultracentrifugation (AUC) is a method for characterisation of macromolecules and their complexes. It can be applied to homogeneous and heterogeneous mixtures of proteins, carbohydrates and nucleic acids. For systems with well-defined stoichiometries it can provide weight values for individual species and sedimentation coefficient values, which combined provide a measure of shape. The Oxford centre has two AUCs: an XL-A providing absorbance optical detection and an XL-I providing absorbance and interference optical detection. The samples can vary from ~2kDa up to 100MDa and the absorbance (wavelengths 190-800nm) and interference means that optical properties of different molecules can be harnessed to their specific detection.