Ion nitriding of steels is a process widely used on steels because it can give the surface important improvements in their tribological and mechanical properties attributable to the iron nitridings formation. On the other hand, on stainless steels, this improvement can normally be obtained by sacrificing their resistance to the corrosion. Nevertheless, several years ago, a new type of phase known as expanded austenite on austenitic stainless steels has been developed, a phase that have shown a high micro-hardness conserving at the same time the resistance to corrosion. In this work, we study the ion nitriding conditions to obtain this type of phase on stainless steel AISI 316L. We have found that the expanded austenite was completely developed after only 30 min of ion nitriding. We obtain images of the surface treated, showing the austenitic nature of the phase, with slip bans that confirms the stress state originated in the expansion of the original austenitic lattice during the ion nitriding process. Further studies of thermal treatment at different temperatures have shown that this expanded austenite developed on AISI 316L is stable for 40 h at temperatures of 300 degreesC, conserving their structure, X-ray diffractogram and a micro-hardness between 1200 and 1400 HV0.025. (C) 2004 Elsevier B.V. All rights reserved.