In the study of iron catalysts, careful passivation is necessary for study of microstructure by ex situ analytical techniques. The passivation procedure used in our study consists of heating the sample in He at the reaction temperature, cooling to room temperature and introducing small amounts of O-2 (< 1%) in a flowing He stream. A properly passivated sample shows no more than a few nm of surface Fe3O4 on alpha-Fe, when examined in a high resolution TEM. Proper passivation is also characterized by an exotherm of no more than 2-3 K. We show that a Fischer-Tropsch catalyst carbided in CO will show substantial amounts of magnetite, if exposed to air without proper passivation. Such surface oxidation may cause errors in determining the relative amounts of the magnetite and carbide phases in Fischer-Tropsch catalysts, which are important for proper identification of the catalytically active phase.