Numerous investigations have used the ionmimplantation of reactive elements (RE) such as Y or Ce, to study their effect on the growth of external oxide scales on alloys. Ion implanatation has, nevertheless, some specific limitations, especially in Al2O3-forming alloys. The most notable limitation occurs at temperatures > 1000-degrees-C where, owing to the shallowness of implanatation, any effects of the ion-implanted RE are short-lived and differ significantly from those observed for an RE alloy addition or an RE oxide dispersion. Additionally, in alumina-forming alloy systems, particularly beta-NiAl, implanted Y stabilizes the first-forming, metastable theta-Al2O3. The retention of the theta-Al2O3 scale on Y-implanted substrates is a chemical effect of the high concentration of Y near the substrate surface and is not a result of the implantation process itself. The loss of the RE effect at high temperatures and long times for implanted alloys is related to the outward diffusion of the RE and the formation of RE-rich oxides near the gas interface of the scale.