Pyrophoric iron powders at different stages of stabilization by oxygen (0-8% by weight) were annealed at 255 or 500°C up to 571 h. During this procedure the magnetic moments first decreased by some per cent and then increased above the initial values. The shape of the magnetic moment versus oxygen uptake curves remained unchanged. The increase of the magnetic moments as a function of time is correlated with a decrease of the specific surface area of the powders. The Mössbauer spectrum of partially stabilized powders is different from that observed on completely stabilized powders and shows a hitherto unknown magnetic iron oxide while X-ray investigations suggest this oxide to be a special iron deficient Fe 3O 4 (spinel) containing vacancies on the octahedral sites distributed at random. The spins in the oxide should then be coupled in an antiparallel way to the underlying iron core in order to explain the measured magnetic moments.