In situ surface X-ray diffraction was used to identify the detailed structure of the passive film that forms on (001)- and (110)-oriented iron single crystals in a berate buffer solution at +0.4 V vs. mercurous sulfate reference electrode, a high passive potential; The passive film is a new phase: a spinel with a fully occupied oxygen lattice, octahedral site occupancy of 80 +/- 10%, tetrahedral site occupancy of 66 +/- 10%, and an octahedral interstitial sire occupancy of 12 +/- 4%. The passive film forms with an epitaxial relationship to the substrate iron; for growth on Fe(001), film(001)parallel to Fe(001) and film[1 (1) over bar 0]parallel to Fe[100], while for growth on Fe(110), film(111)parallel to Fe(110) and film[1 (1) over bar 0]parallel to Fe[100]. The in-plane lattice parameter for the passive film (the LAMM phase) is 8.39 +/-0.01 Angstrom for growth on both faces, and the out-of-plane lattice parameter is 8.25 +/- 0.1 Angstrom [Fe(001)] and 8.42 +/- 0.1 Angstrom [Fe(110)]. The passive film forms a nanocrystalline microstructure with numerous defects. Specifically, the grain size is 50-80 Angstrom in-plane and about 30 Angstrom out-of-plane. There is a small mosaic spread of 2.5 to 4.1 degrees and a high density of antiphase boundaries and stacking faults. The structure of the film determined in situ was found to be identical to that found for an emersed sample, indicating that the high potential film studied here is stable on removal from the electrolyte. Some of the implications of the film structure on passivity are discussed.