The internal reflection of an excited X-ray waveguide mode in a synthetic nanostructure, defined by electron-beam lithography, has been measured. In this device, the X-ray beam is first coupled into a conventional vertical thin-film waveguide structure and then reflected laterally at the quasi-one-dimensional edge of the waveguiding layer. The reflectivity of the quasi-one-dimensional interface has been recorded under simultaneous excitation of the (vertical) waveguide mode. The experiment constitutes an important step towards the production of a coherent nanometre-sized X-ray point source by two-dimensionally defined waveguide structures