Samples of the austenitic stainless steel AISI 316 nitrided at low temperature in a low-pressure r.f, plasma have been examined by scanning electron microscopy (SEM) and secondary ion mass spectrometry (SEM), with a view to determining the composition of the treated surface layer. With suitable sample preparation SERI can provide clear images of details of the treated layer that facilitate a relatively direct measurement of layer thickness. We show explicitly that the sputter rate in a SIMS depth profile is constant for this material. Hence the depth scale can be easily calibrated, so that SIMS is a convenient method of measuring the thickness of the treated layer, The SIMS depth profile suggests that the small proportion of carbon in the alloy is pushed ahead by the incoming nitrogen and piles up at the interface between the treated layer and the hulk material. The sputter yields of the main components of the alloy vary differently with the concentration of nitrogen, whereas that of carbon appears to be unaffected by nitrogen concentration.