Magnetite is a redox active Fe oxide common in most soil and sedimentary environments. In this study, we investigated magnetite transformations under extreme alkaline conditions (0.1–2 mol l−1 NaOH) similar to those found under high-level radioactive waste storage tanks at the Hanford site in the State of Washington, USA. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results showed that magnetite was transformed into maghemite and goethite, and the XRD peak intensities for goethite increased with NaOH concentration. Goethite was presumably formed through reconstructive dissolution/crystallization reactions that can be written as: Fe3O4 + OH− + H2O = γ-Fe2O3 + Fe(OH)3−; 2Fe(OH)3− + ½O2 + 2H+ = 2α-FeOOH + 3H2O. Some of the newly formed maghemite may also redissolve to form goethite at greater NaOH concentrations.