Structural changes of the Fe-Mo, Fe Ti and Fe- Nb binary alloys have been investigated by means of electron microscopy and X-ray diffraction. A tweed structure representing modulation of the composition always precedes the Laves phase precipitation. The latter fact allows confirmation that the Laves phase precipitation proceeds by the spinodal mechanism. A tweed structure formation results in softening of the Fe-Ti and Fe-Nb alloys; the hardness of the Fe-Mo alloys is unchangeable.