Abstract Mössbauer studies of the magnetic properties of synthetic feroxyhites (δ´-FeOOH) of varying crystallinity showed that large crystallites are ferrimagnetic, probably due to uncompensated spins associated with defects. Heating of these crystallites led to a reduction of their magnetic moment, presumably because the number of defects was reduced during heating. Small crystallites of feroxyhite were found to be speromagnetic, having a magnetic ordering temperature between 80 and 300 K, in contrast to a ferrimagnetic Curie temperature of about 450 K for large crystallites. A speromagnetic component was also found in the spectra of the large crystallites, probably due to the surface atoms.