Using surface x-ray diffraction we have investigated the geometric structure of the interface between ther-mally grown MgO layers and Fe(001). The MgO/Fe(001) interface is part of the Fe/MgO/Fe junction, which has become a prototype system in the study of the tunneling-magnetoresistance (TMR) effect. For all samples studied in the MgO coverage range between about 0.35 and 4.6 ML we find clear evidence for the presence of a substoichiometric FeO layer between the bulk Fe crystal and the MgO adlayers. The partial oxidation of the Fe(001) surface takes place during deposition of the first MgO monolayer and approaches a concentration limit, where about 60% of the Fe(001) hollow sites are occupied by O ions. The formation of a bulklike sixfold-coordinated Mg coordination at the MgO/O/Fe(001) interface might be accounted for stabilizing the interface structure, in which several FeO distances are strained by up to (10% with respect to their bulk analog. The presence of the strained interfacial FeO layer is likely to have considerable consequences on the magnitude of the TMR effect.