This master thesis examines the kinetics of expanded austenite formation on AISI 316L by in-situ observation of the development of its X-ray diffraction pattern during gaseous low temperature nitrocarburization. This novel method allows for collection of detail information about the expanded austenite formation process with a relatively fine temporal resolution. Furthermore, the influence mechanisms of several parameters relevant to the thermochemical treatment process are investigated. Treatment duration, processing temperature, gas configuration, sample surface roughness, the method of surface activation and the structural condition of the untreated sample are taken into consideration. Observing the X-ray intensity absorbed by the evolving expanded austenite layer it was possible to obtain diffusion coefficients valid for the initial stages of formation and compare those coefficients for different process setups. Activation energy values for interstitial diffusion were identified by fitting the respective diffusion coefficients in Arrhenius type plots. Depending on the microstructural condition of the specimens, the method of surface activation and the reaction gas configuration the activation energy for interstitial diffusion turned out to lie between 89kJ/mol and 201kJ/mol. Supporting ex-situ analysis tools delivered indications for an influence of surface roughness and thermal pre-treatment on the resulting expanded austenite layer thickness and the associated specimen hardness. The attempt to find out more about chromium nitride formation resulted in the realization that the Bragg reflections of CrN-peaks are not suitable for examining this process, because they appear at a stage of advanced precipitation. However, the observed absorption of X-rays by chromium nitride and the associated shielding of the expanded austenite reflections provide a promising method for further research on this topic.