Model molecules, designed to simulate a chain fold, self-assemble into crystalline monolayers at the air-water interface, the structure of which has been determined, in situ, by synchrotron grazing-incidence X-ray diffraction. The two-dimensional packing arrangement in such monolayers is consistent with a structure composed of ribbons containing translationally related molecules adopting an inverted U-shape. The distance between these molecules along the ribbon is optimum for creating an additional chain fold, leading to the possibility that a polymeric molecule will undulate on the water surface.