The surface compositional changes in the surface layers of Cu₃Au(1 0 0) have been studied by medium-energy ion-scattering spectroscopy (MEIS). Using 100 keV N⁺ ions in a MEIS experiment, single atomic layer depth resolution was obtained. At a temperature starting at about 500 K, the Au atoms in the surface layer of Cu₃Au(1 0 0) apparently began to move to the second layer with increasing temperature. The atomic concentration of Au in the first layer as measured by MEIS then decreased from 50% at room temperature (RT) to 32% at 720 K. This decrease in the atomic concentration of Au atoms in the first layer was continuous, rather than abrupt at the critical temperature. Moreover, it was found that the compositional change between the first and the second layers was reversible at temperatures up to 720 K. The atomic concentration of Au atoms in the third layer remained constant over the temperature range from RT to 720 K. The difference between this result, where the first layer Au concentration decreases, and the previous low-energy ion-scattering spectroscopy (LEIS) measurements which show a relatively constant Au concentration, lies in the production of surface vacancies and adatoms at elevated temperatures. These defects have no significant effect on yield in LEIS experiments but are seen using MEIS.