The x‐ray standing‐wave method is a powerful technique for accurately locating the position of impurity atoms at surfaces, interfaces, and in bulk crystals. Using UHV to prepare crystal surfaces andinsitux‐ray experiments, we have addressed a number of problems in surface physics. Specifically the location of As atoms on Si(111) has been accurately established and has allowed for the first time a direct test of theoretical, density functional, total energy minimization, calculation of atom positions. For As, with only nearest neighbor relaxations, there is excellent agreement between theory and experiment. For Ga on silicon (111) the situation is much more complex. At low coverages (∼1/3 monolayer) the adatomT4site for Ga has been accurately measured. X‐ray standing waves were used to establish the vertical position and the in‐plane registration site of Ga to the substrate silicon atoms was established both by standing waves and tunneling microscopy. At higher coverages, Ga occupies substitutional sites in the top half of the Si(111) double plane, but contracted inwards by about one‐half of the distance between the Si(111) double planes. The consequences of the large surface strain generated by this contraction is discussed in the context of the standing wave and tunneling microscopy results. In addition, the atom positions that prevail when both Ga and As atoms are deposited on Si(111) are described.