© 2018 Elsevier B.V. The colloidal nanoparticles FemOn–SiO2 have been obtained using the sol–gel method. Based on the data obtained with transmission electron microscopy and dynamic light scattering, a lognormal size distribution of nanoparticles with an average size of 20–30 nm have been constructed. The shape of Mössbauer spectra implies that they have the characteristic relaxation time of the order of 10–9–10–10 s that corresponds to the grain size of about 10 nm. X-ray diffractometry indicates that the crystalline phase of the particles is magnetite. Hysteresis loops have been traced with a vibration sample magnetometer and the demagnetization curve of anhysteretic remanent magnetization measured with a SQUID magnetometer. Theoretical analysis of magnetization data based on a model of magnetostatically interacting superparamagnetic nanoparticles confirms the presence of both separate superparamagnetic magnetite particles (less than 20 nm in size) and those organized into clusters with an average size of about 30 nm.