Oxidation of iron has been studied in air at temperatures from 450 to 600°C by in situ measurements of Mössbauer absorption spectra combined with thermogravimetry and scanning electron microscopy. The oxidation proceeded in two succesive stages from α-Fe to Fe₃O₄ and to α-Fe₂O₃. The first stage of oxidation was well expressed by a first order rate law and the second stage by a parabolic rate law. The first stage of oxidation was attributed to a two-dimensional, heterogeneous growth of Fe₃O₄ on the iron surface and the second stage to a diffusion controlled process. The activation energies were 46.6 for the first stage and 48.4kcal/mol for the second stage. Thermogravimetric results were shown to follow the same rate laws obtained by Mössbauer spectroscopy. Observations by scanning electron microscope supported the interpretations. Mössbauer spectroscopy showed that the total iron contained in the detected constituents decreased with the increase in measuring temperature. This was determined to be due to the temperature dependence of the Mössbauer absorption intensity being higher for the oxidized Fe₃O₄ than for bulk Fe₃O₄. These observations are ascribed to the marked difference in the force constsnt, or the Debye temperature, between both kinds of Fe₃O₄