Metallurgical and mechanical responses to the introduction of carbon and/or nitrogen have been investigated for alloys 16NiCrMo13 and 23MnCrMo5 through atmospheric pressure carbonitriding, carburizing and austenitic nitriding. Treatments were performed at 1173 K using CO − H 2 and/or NH 3 based atmospheres. As-quenched hardness is studied in terms of solid solution interstitial content, which is estimated from electron probe micro-analyses and thermodynamic simulations. This aims to quantify high-temperature precipitation of nitrides, which is accompanied by consumption of nitrogen in solid solution. Plots of as-quenched hardness versus the square root of the total content expressed in atomic fraction of interstitial elements, i.e. the sum of solution carbon and nitrogen, show the complementary character of these elements in determining the mechanical properties of the as-quenched alloys. Tempering of carbon-nitrogen martensite-for both carbonitriding and austenitic nitriding-led to lower hardness loss when compared to the same total content of interstitial carbon in martensite. Transmission electron microscopy analyses highlight precipitation of fine Fe 16 N 2 zones in the nitrogen-rich depth.