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Surface Mössbauer measurements have been carried out for 38NCD4 steel samples, ion implanted with nitrogen at different doses and currents. It has been found that Fe-N compounds form in a wide range of compositions as aggregates in the steel matrix: from the unaffected base metal to the external surface, steel layers follow one upon the other in which first Fe2+yN with 0.3 ⩽ y < 2.7, then Fe2+xN with 0 ⩽x < 0.3, and finally austenite containing ∼10 at. % nitrogen, predominantly form. Dose decrease decreases the thickness of the above layers, while current variations produce differences in the relative amounts of the various Fe-N compounds.