An expanded austenite layer is formed on the surfaces of austenitic stainless steels that are nitrided under low-temperature plasma. This S phase is an iron alloy metastable phase supersaturated with nitrogen. We have identified a similar expanded ferrite or ferritic S phase for nitrided ferritic (BCC) stainless steels. Samples of austenitic AISI 304L and AISI 316L and ferritic AISI 409L stainless steels were plasma-nitrided at 350, 400, 450 and 500°C, and the structural and corrosion characteristics of the modified layers were analyzed by X-ray diffraction (XRD) and electrochemical tests. For the austenitic AISI 304L stainless steel, the results showed that a hard S phase layer was formed on the surface, without corrosion resistance degradation, by using low plasma temperatures (350 and 400°C). A similar behavior was observed for the austenitic AISI 316L stainless steel: the modified layers formed at 350 and 400°C were constituted mainly by the S phase. Plasma-nitriding treatment of the ferritic AISI 409L stainless steel caused the formation of a layer having high amount of nitrogen. XRD measurements indicated high strain states for the modified layers formed on the three stainless steels, being more pronounced for the ferritic S phase.