The microstructure of Fe-15 wt% W and Fe-18 wt% W alloys has been studied using methods of transmission electron microscopy and X-ray diffraction. It has been shown that in the region of a solid solution the separation microstructure is formed. During isothermal aging at 600degreesC the separation microstructure is dissolved and before precipitation of the stable Fe2W phase an unknown metastable phase is formed and then dissolved. The conclusion has been made that a solid solution region in the Fe-W system is actually the region in which a tendency toward separation takes place. (C) 2004 Kluwer Academic Publishers.