The morphology of the periodic microstructure and its influence on the process of Fe-Ti alloy decomposition were investigated by means of transmission electron microscopy, field-ion microscopy and X-ray diffraction. The tweed structure is a system of long cylindrical columns enriched in titanium and situated along the elastically soft directions of the Ti-depleted Fe-matrix. The tweed structure forms in the monophase region of the phase diagram as a result of the tendency of Fe-Ti alloys to separation and dissolves during aging before the precipitation of a new Fe 2 Ti phase.