Plasma immersion ion implantation (PIII), although originally developed as an alternative non-line-of-sight ion implantation technique, is also capable of producing structures typical of thermochemical processing. In this work, the structure and properties of PIII and plasma nitrided AISI 316 austenitic stainless steel were characterized in order to establish the similarities and differences between PIII and plasma nitriding. A distinct difference between PIII and plasma nitriding has been identified with respect to the degree of nitrogen supersaturation of the austenite that can be achieved. It is proposed that the extent of supersaturation and the resultant microstructure are primarily controlled by the nitrogen mass transfer mechanism. For plasma nitrided stainless steel, a surface iron nitride layer is formed and limits the nitrogen content to less than 20 at. %. For PIII, where nitrogen is also directly implanted below the surface, nitrogen contents near the theoretical limit of 50 at. % can be achieved.