[en] Effect of sample layer on properties of iron(III) oxide nanoparticles prepared by isothermal decomposition of the FeC₂O₄·2H₂O powder has been studied in static air conditions at reaction temperature of 175 grad C using ⁵⁷Fe Moessbauer spectroscopy, XRD, TEM, and BET surface area measurements. In-field Moessbauer spectroscopy was applied for distinguishing and quantification of amorphous and nanocrystalline (γ-Fe₂O₃, [α-Fe₂O₃) iron oxides. Independently on the layer thickness, amorphous iron(III) oxide nanoparticles (1-2 nm) are produced as the primary decomposition product. Above a critical thickness, amorphous nanoparticles crystallize to nanocrystalline superparamagnetic maghemite (γ-Fe₂O₃, 5-8 nm), which process is accompanied by an increase of the sample temperature. With increasing thickness of ferrous oxalate layer, the crystallization of primarily formed amorphous Fe₂O₃ nanoparticles starts later with longer duration. The maghemite samples prepared above a critical weight reveal contrast properties suitable for application in magnetic resonance imaging. Below a critical thickness, however, the crystallization process proceeds very slowly without any indications of an increase of sample temperature. Hematite with a large surface area and excellent catalytic properties was clearly identified as the only crystallization product. The possibility of unique monitoring of the crystallization of amorphous phase through an increase of the sample temperature is demonstrated on other solid state reactions including thermal conversions of iron(II) acetate and Prussian Blue. (authors)