Measurements have been made on the effects of ion-beam milling on the surface of Cu-Ga alloys using a technique in which the optical reflectivity of an ion-bombarded surface is compared with a nonbombarded region. The differential-reflectometry method has a sensitivity to less than a 1% change in composition in the surface layer and probes the surface to a depth order of 50-100 Å corresponding to the optical skin depth. The results have been correlated to changes in the peak heights of Cu and Ga measured by Auger electron spectroscopy. In this way it has been found that negligible selective sputtering of the Cu and Ga occurs when the alloys are ion bombarded with 2-keV argon ions. The ratio of the AES peak heights due to Ga and Cu from the oxide differed from that of the alloys by about a factor of 2.