An ion-sputtered, oxygen-exposed, polycrystalline palladium surface has been studied using ion scattering spectroscopy (ISS) before and after exposures to H-2 and CO. A room-temperature exposure to O-2 for 1 min at 1 x 10(-7) Torr does not yield chemisorbed oxygen in the outermost layer of atoms according to the ISS data. However, subsequent exposure to H-2 or CO induces subsurface oxygen formed during the O-2 exposure to migrate to the surface under a chemical driving force and populate the outermost atomic layer where it is detected by ISS. This study demonstrates that a reductive treatment can increase the oxygen content of a surface and provides a useful method for examining subsurface oxygen.