Small particles of iron oxides, nominally 7 nm and 13 nm in diameter, have been studied by Fe-57 Mossbauer spectroscopy in the temperature range 4 K to room temperature. The Mossbauer spectra show, at high temperature, relaxational behaviour in agreement with the superparamagnetism, expected. At low temperature the spin motion becomes slower and, finally, blocked. There are also indications that the material is non-stoichiometric. (c) 2005 Elsevier B.V. All rights reserved.