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.