In this work, the response of AISI 304 stainless steel after having been implanted with 10(17) ions/cm(2) doses of Y and Er at an acceleration potential of 150 keV is examined. Computational estimates of the implanted profile have been performed using the PROFILE code. The nature of the outermost surface of the steel, i.e. the passive layer and onwards is studied by means of Auger Electron Spectroscopy as well as X-Ray Photoelectron Spectroscopy techniques, supported by thermodynamics calculations. It is shown that the incorporation of both reactive elements in the steel surface brings about different changes in the nature of their respective passive layers. Both implanted ions induce oxidation of the base material to a certain extent, probably due to the implantation process itself giving rise to Cr2O3 and FeO and/or Fe-OH species, together with different RE oxides as well as Y(OH)3.