The corrosion behaviour of both as nitrided and nitrided then annealed 316 austenitic stainless steel has been investigated. Results show that low temperature plasma nitriding can, to some extent, enhance the corrosion resistance of 316 steel, and this can be attributed to the high nitrogen content in the nitrogen S phase layer. However, when annealed at certain conditions, the corrosion resistance of the S phase layer would be impaired. This is mainly because thermodynamically the S phase is a metastable phase, which will decompose into stable chromium nitride and the alpha phase if the kinetic conditions (temperature and time) are favourable. Clearly, the improved corrosion resistance conferred by low temperature plasma nitriding can be retained provided the S phase layer remains untransformed under service conditions. (C) 2003 IoM Communications Ltd. Published by Maney for the Institute of Materials, Minerals and Mining.