Sub-surface oxide layers can be synthesized by implantation of mixed metal-gas ion beams formed by a modified vacuum are ion source. The particle current fractions of the metal and gas ion components in the beam can be controlled from 100% metallic up to approximately 80% gaseous depending on the ion source operational parameters. We have carried out some exploratory investigations of surfaces modified by co-implantation in this way, particularly of stainless steel ion implanted with Al + O and (separately) Zr + O. Due to the ubiquitous surface layer of native oxide present on the steel substrate, additional oxygen can be recoil-implanted, and the resultant implantation can be different from that expected-the effect of the energetic oxygen ion beam is clouded by the recoil implanted oxygen. To help elucidate this complicated scenario, we have carried out a number of metal-gas co-implantation experiments in various kinds of materials. Here, we briefly describe the ion source modifications and the co-implantation process, and report on the state of our understanding of the overall sub-surface layer structure and the tribological benefits of this kind of surface modification. (C) 1998 Elsevier Science S.A.