Iron specimens passivated in borate buffer solutions, pH 8.4, have been investigated by transmission electron microscopy. The observed diffraction patterns are characteristic of spinel‐type iron oxide in all specimens independent of the passivation potentials. The orientation relationship between the oxide and iron substrate has been determined to be typical of orientation relationships between fcc and bcc materials,namely, Bain, Kurdjumov‐Sachs, and Nishiyama‐Wasserman types. Specimens have also been examined in a hydrated environment using an environmental cell in a high voltage electron microscope. Changes in the diffraction spots from the anodic passive films have been found during observations in the hydrated and subsequently dehydrated environments. The suggested structure of the passive film is a fine grained crystalline iron spinel oxide consisting of a varying iron concentration from the metal/oxide interface to the oxide/solution interface.