Mössbauer spectroscopy has been used to study the phase composition of ultra-small iron-based particles obtained by gas-phase synthesis in an inert gas atmosphere followed by surface passivation at a low partial pressure of oxygen. The main phases in the particles are the bcc iron, magnetite, and ferric hydride. The proportion of ferromagnetic phases decreases, while the oxide shall thickness increases (from 1.6 to 3 nm) in particles of smaller dimensions. The results can be used for estimating the level of magnetic hysteresis in nanoparticles with allowance for the paramagnetic phase fraction. © Pleiades Publishing, Ltd. 2007.