The out-diffusion of toxic Ni ions from NiTi (Nitinol), often used for biomedical applications, can be strongly reduced using oxygen plasma immersion ion implantation (PIII). Non-radioactive isotope markers, Ni-60 and Ti-46, were implanted at 180 keV into NiTi prior to the oxygen implantation with fluences between 0.4 x 1016 and 4 x 10(16) at./cm(2). Implanting oxygen ions by PIII in the temperature range of 400-550 degrees C leads to a surface oxide layer consisting of pure TiO2. The results prove that Ni cations are the mobile species, while Ti is immobile during the oxygen insertion. (c) 2008 Elsevier B.V. All rights reserved.