The near surface structure and nitrogen concentration of the low-temperature low-pressure ion-nitrided stainless steels (SS) was studied by using X-ray diffraction (XRD), transmission electron microscopy (TEM), nuclear resonance broadening (NRB) and microhardness techniques. The surface nitrogen content as determined by NRB was found to increase with nitriding time such that at long nitriding times the surface nitrogen concentration was higher than for any equilibrium nitride in the FeN system. Nitrogen contents were slightly higher for type-304 than for type-316 stainless steels. Simultaneously with increasing surface nitrogen content, a strong shift and broadening of X-ray diffraction peaks occured. In the surface of the nitrided layer expanded austenite as well as ε-phase analogous to ε-martensite is formed. At long nitriding times (high nitrogen surface contents) the structure of the surface corresponds to cubic MN1-x nitride. At intermediate nitriding times (and nitrogen contents) possibly some ε′-nitride is also formed. TEM study of the surface layer showed that after long nitriding times the expanded austenite with occasional weak primitive lattice reflections was the dominating phase and the hexagonal ε-phase was habited as thin platelets on the (111) planes of the nitrogen supersaturated austenite. The hardness of the compound layer can be as high as 25 GPa. The high hardness is suggested to result from nitrogen supersaturation, high dislocation density and thin platelets of ε-phase in the surface of the compound layer.