Mössbauer spectroscopy was applied to study the structure of iron-containing particles of atmospheric aerosols. From the temperature dependence of the Mössbauer spectra it was concluded that iron appears in atmospheric aerosol mostly in the form of ultrafine, superparamagnetic particles of Fe2O3. The Mössbauer technique permits not only the identification of the chemical compound in which iron appears but also the evaluation of the size of particles in which it is contained. The analysis of the quadrupole splitting distributions provide information on the differences of particle sizes in aerosol collected in different geographical locations. The concentration of iron in air has been calculated from the experimental spectra. The seasonal variation of iron concentration at selected sampling sites is discussed.