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S phase layers produced on AISI 316 stainless steel by low temperature plasma nitriding and plasma carburising were examined in order to investigate their mechanical and chemical properties. Cylindrical disc specimens were used for testing hardness, pin on disc wear and electrochemical corrosion. Toroidal specimens were used for the fatigue and fretting fatigue tests. Both plasma nitriding and carburising processes created S phase layers on the surface of AISI 316 and greatly improved the steel's tribological properties. SEM showed that more precipitation of nitrogen occurred in the plasma nitrided specimens and GDS revealed that the nitrogen in the nitriding specimens did not penetrate as far into the substrate as the carbon in the carburising specimens. This is due to the nitrogen bonding easily with chromium in the surface layer which obstructs the diffusion of nitrogen into the substrate. The hardness tests showed that the nitriding layer was harder than the carburised layer but was more brittle and did not extend far into the substrate whereas the hardness of the carburised layer reduced gradually to a depth of 10 μm. The nitrided layer did better in the abrasion tests whereas the carburising layer performed better in the fatigue tests. The importance of these discoveries is discussed.