A method is described for determining the structure of a surfactant complex formed at an immiscible liquid-liquid interface. The surfactant complex forms and crystallizes at the interface between an aqueous phase comprising a divalent metal salt and a non-aqueous phase comprising an anionic surfactant. The non-aqueous phase may be in the form of a droplet surrounded by the aqueous phase. The structure of the surfactant complex is determined by X-ray crystallography.