Ferrihydrite is a generic term for various poorly ordered Fe(III) oxyhydroxides which are naturally occurring as nanocrystals and are believed to constitute the ferric core of ferritine, the main iron storage protein in biological systems. Unlike other iron oxides, the exact structure and composition of ferrihydrite is still a matter of debate. In this work, we have prepared and characterized the two main forms of ferrihydrite referred to as 2-lines and 6-lines, on the basis of the number of reflections observed in the (X-ray) diffraction pattern. Thermal and textural properties have been studied: structural characterization has been performed by X-ray diffraction, transmission electron microscopy and X-ray absorption spectroscopy (EXAFS and XANES). The structure of the two forms results to be quite similar. The study of the magnetic properties indicates that the small differences between the 2-lines and 6-lines ferrihydrite samples are mainly caused by the different weight of the magnetic spins located on the particle surface, related to the different nanoparticles mean size. (C) 2008 Elsevier B.V. All rights reserved.