A standing X-ray wave is set up by using a Bragg reflection from the crystal surface. The incident and reflected beams create the standing wave which will extend out of the surface. The intensity of the standing wave at a particular position can be changed by altering the reflection angle, or the photon energy. The intensity of an Auger (see AES) or fluorescence (see EDX) peak from a molecule attached to the surface is then monitored. The variation of the intensity of the peak can provide information regarding the adsorption site and thus provides information complementary to SEXAFS and XPD). XSW has the advantage of being able to work in non-UHV conditions and to investigate buried interfaces, but a high degree of crystal perfection is necessary. Changing the photon energy means that a synchrotron radiation source is required.