The study is devoted to the thermal decomposition of the iron oxyhydroxides: gamma-FeOOH (lepidocrocite), alpha-FeOOH (goethite) and Fe5HO8.4H(2)O (ferrihydrite). The changes in the crystal structure, in the phase composition and in the dispersion degree, occurring during thermal treatment have been followed by means of Mossbauer spectroscopy, X-ray diffraction, infrared spectra, transmission electron microscopy (TEM), DTA/TG analysis and by measuring the specific surface area. The schemes of thermal dissociation of oxyhydroxides have been outlined on the basis of the obtained results. Some relaxation effects of nano-sized particles of the intermediate products have been observed in the course of this investigation and described respectively. Depending on the precursor, from which alpha-Fe2O3 has been obtained, it possesses various morphological (needle-like or spherical shape) and dispersion (particle size and specific surface area) features. The obtained results enable forecasts with respect to the optimal final product to be used as initial material for preparing heterogeneous catalysts and magnetic materials.