The main purpose of the present work is to study the effect of a high nitrogen content (1 wt% N), on the microstructural evolution of a Cr-Mn austenitic stainless steel aged over the [400-900 degrees C] temperature interval. Thermal treatments carried out between 700 and 900 degrees C lead to the decem position of the nitrogen supersaturated austenitic matrix by discontinuous precipitation of Cr2N particles. The microstructural features of the reaction are described and analysed. In the present case, the cellular precipitation of Cr2N is a peculiar and complex phenomenon which involves two diffusion mechanisms : the diffusion of an interstitial element (nitrogen) and the diffusion of a substitutional one (chromium). The nucleation of the discontinuous precipitation arises from a reduction of the surface energy of the precipitates. Furthermore, the precipitation growth is a non-steady state process, because the reaction is governed at first by the intergranular diffusion of chromium, and then tends to be controlled by its bulk diffusion. Consequently, the features of this discontinuous precipitation do not fit in with the assumptions of usual theories, which have been established for binary substitutional systems that transform in steady state conditions. This discontinuous precipitation brings about a slight hardening. Then, the hardness of the aged samples can be described by an additive relationship between the hardness of the precipitation cells and that of the untransformed matrix. Beside the discontinuous precipitation of Cr2N, sigma phase forms with significant volume fractions.