Alloys of the FeMnAlC system have been used for the manufacture of components for different engineering applications, such as in the aircraft and chemical industries. The best combination of properties for these applications has been obtained so far by a conventional aging treatment. In this paper the possibility of further improvement in mechanical properties of an alloy of this system by a two-step aging treatment and the stability of the resultant microstructure at service conditions and temperatures up to 500 °C are presented and discussed. It is shown that the resultant microstructure enables the steel to be used in applications involving continuous exposition at 400 °C for 500 h, even when subjected to simple stresses up to 60% of the σ0.2 yield stress.