Endocytosis plays a central role in allowing the selective transport of molecules, via membrane-derived vesicles, into cells. Vesicle formation is controlled by a complex network of proteins, some of which form a protein coat around the vesicle, giving these structures the name, ¿coated vesicles¿. We wish to understand how this network is formed in the case of clathrin-coated vesicle formation. To achieve this we need to visualise and quantify the nature of the interactions between clathrin and its accessory proteins. Since many accessory proteins adopt an extended conformation, visualising them bound to clathrin cages is problematic using other structural techniques. We aim to solve this problem using contrast variation SANS. Using auxilin binding to clathrin as a model system, we aim to develop a general approach to resolving accessory protein structures bound to clathrin cages.