The microstructural development of Fe–N–C compound layers resulting from nitrocarburising and heat treatments applied to Fe–N and Fe–C alloys was investigated. Slow cooling or secondary annealing, instead of quenching, after the end of the nitrocarburising treatment causes decomposition of γ-Fe[N,C] and ∊-Fe3(N,C)1+x in the compound layer. The resulting decomposition microstructures were analysed by, in particular, light microscopy and electron backscatter diffraction, and X-ray diffraction and electron probe micro-analysis, revealing their constitution and crystallographic properties. The results are discussed in comparison to data obtained from undecomposed, i. e. quenched, specimens and to data reported in the literature.