In the course of X-ray photoemission studies on the oxidation of metals and alloys and on bulk oxides, we found that in addition to physical sputtering Ar+ ion bombardment can in many cases reduce an oxide to a mixture of the original oxide, lower oxides and metal. The effect is even more pronounced in systems containing hydroxyl groups which are readily destroyed by the ion beam. Specific examples for oxidized cobalt, nickel and iron surfaces and their bulk oxides and hydroxides are given. The relative reduction rates of CoII and FeIII in CoFe2O4 are also examined. From these observations, it is clear that any depth compositional profiling using ion sputtering in conjunction with Auger or X-ray photoelectron spectroscopy should be treated with extreme caution. The mechanism for the chemical changes induced by ion bombardment is briefly discussed. © 1978.