⁵⁷Fe Mössbauer spectroscopy is demonstrated as an effective tool in the identification, magnetic and structural characterization of iron(III) oxides formed by thermally induced oxidative decomposition of FeC₂O₄.2H₂O and by the secondary crystallization of amorphous Fe₂O₃ nanopowder. AFM measurements revealed that the primary amorphous nanoparticles are between 1 and 3 nm, in accordance with the large surface area of 250 m²/g as determined by BET analysis. The temperature depended Mössbauer measurements of amorphous Fe₂O₃ showed the magnetic transition temperature lower than 70 K with the sharp evolution of the spectral lines between 53 and 76 K as typical for ultrafine and strongly interacted particles. As the primary crystallization product at low temperatures, γ-Fe₂O₃ (maghemite) was identified by XRD. The broad distribution of magnetic fields in Mössbauer spectra illustrates the broad size distribution of maghemite nanoparticles. The thermally stable α-Fe₂O₃ (hematite) can be produced by the direct crystallization of amorphous phase at higher temperatures or by the structural transformation of the primarily formed maghemite at lower temperatures. Particles of hematite formed at 200°C reveal the Morin transition temperature suppressed below 20 K corresponding to the size about 20 nm.