We have performed in situ X-ray absorption (XAS) experiments on single crystalline iron-doped nickel oxide (Ni1-xFex)(1-delta)O, x = 4.5%, at the FeK-edge at high temperatures (1273 K less than or equal to T less than or equal to 1473 K) and under defined oxygen activities to study the redox behavior of the dopant iron in this system. To be able to correlate the absorption edge position with the absorber mean charge, a XAS shift scale based on defined iron reference compounds (Fe3O4, Fe2O3, SrFeO2.86) was elaborated. It was possible to correlate our XAS results with a quantitative defect model. based on thermogravimetric measurements, which inter alia consists of tetravalent iron ions tetrahedrally coordinated by four cation vacancies. We could confirm this model and find evidence for the existence of Fe4+ in this system.