The structure and magnetic properties of nanocrystalline Fe-O alloys produced by high-energy ball milling and subsequent low-temperature annealing were investigated. The Fe2O3, FeO and Fe powders as well as their mixtures were used as starting materials. The structure was studied by X-ray diffraction analysis, Mossbauer spectroscopy and scanning electron microscopy. The magnetic properties were measured in vibrating sample magnetometers at room temperature and 4.2 K. The nanocrystalline composite alloys obtained as a result of the milling contained FeO and alpha-Fe with an average crystallite size of 15-20 nm as well as an amorphous phase, which was identified as a solid solution of oxygen in iron. However, alloys subjected to subsequent annealing contained only alpha-Fe and Fe3O4 with an average crystallite size of about 20 nm. Unlike the starting materials the produced powders had properties which are characteristic of hard magnetic materials. For example, the powder produced by the milling of Fe2O3 + 50% alpha-Fe mixture followed by annealing had the following properties at 300 K: intrinsic coercive force mu(0)H(c) = 0.067 T, remanence B-r = 0.48 T, energy product (BH)(max) = 9 kJ/m(3) (C) 2004 Kluwer Academic Publishers.