While it is established that the main process of alumina scale growth on NiAl-alloys, after a protective layer has been formed, is the diffusion of oxygen through the scale to the metal, there is also a contribution of metal outward diffusion. The diffusion processes, which happens along grain boundaries (GBs), can be manipulated largely by the choice of alloying reactive elements segregating towards GBs. For this study, we applied transmission electron microscopy and atom probe tomography (APT) to analyze oxidized NiAl without additions, with Hf, and with Zr. Due to the high chemical and spatial resolution APT provides, it was possible to observe and even quantify GB segregation of Hf, Zr and Ni. With careful site-specific sample preparation, it was also possible to determine the outward diffusion of Ni, Hf and Zr besides the by four orders of magnitude dominating Al-diffusion, which itself is six orders of magnitude smaller than the O-inward diffusion.