Fluorination of magnetite (Fe3O4) by NH4HF2 was investigated using simultaneous thermogravimetry and differential thermal analysis (TG-DTA), and observing the morphology and phase changes using scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) and X-ray diffractometry (XRD). The results indicate that fluorination with the involvement of oxygen begins at room temperature, peaks at 1784ºC, and is completed at 200ºC with the formation of only (NH4)3FeF6. On heating, (NH4)3FeF6 gradually releases NH4F by the formation of NH4FeF4 at 259ºC, then (NH4)0.l8FeF3 at 327ºC, and finally FeF3 with minor FeF2 at 400ºC due to the partial reduction of Fe (III) to Fe (II). At 550ºC, FeF3 is oxidized to FeOF/Fe2O3.