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The in situ rearrangement of (Cr, M)TCj-type carbides has been observed during ion nitriding of a commercial chromium-carbon alloyed steel (Zl 60CDV1 2). The mass balance concerning nitrogen, carbon and substitutional elements proves that carbides undergo a total transformation into substitutional (Cr, M) N-type nitrides with a simultaneous release of a-iron from the very nitride phase. A detailed transmission electron microscopy microstructural study confirms the analytical results. There are no crystallographic relations between the carbides and their corresponding transformation products. The rejection of carbon from the carbides into the ferrite matrix leads to the precipitation of a network of cementite along the prior austenite grain boundaries.