Fe-Cr alloys with 4, 7, 13 and 20wt.% chromium were nitrided in an NH3/H-2 gas mixture to investigate the microstructure and the growth kinetics of the nitrided zone in dependence on the chromium content. The precipitation morphology of the nitrided samples was investigated with light optical and scanning electron microscopy. The concentration-depth profiles were determined with electron probe microanalysis. The evolution of the thickness of the nitrided zone could be described by a numerical model. As a result the amount of mobile excess nitrogen dissolved in the ferrite matrix and the amount of immo*bile excess nitrogen could be determined.