Metallurgical and mechanical responses of alloys 16NiCrMo13 and 23MnCrMo5 to the addition of carbon and/ or nitrogen were investigated. Diffusion profiles of these interstitial elements were established by atmospheric pressure carburizing, austenitic nitriding, and a sequence of carburizing and nitriding-the carbonitriding. All treatments were performed at 1173 K under CO-H 2 and/or NH 3 based atmospheres. After enrichment, each sample was (i) room-temperature oil-quenched and (ii) immersed in boiling nitrogen prior to (iii) the stress relief treatment. Cross-section hardness profiles were evaluated after each of these steps. Electron probe microanalysis (EPMA) allowed for the determination of both carbon and nitrogen diffusion profiles after quenching. In order to estimate the fraction of nitrides formed during the enrichment of the alloys, these measured profiles were employed in the simulation of local equilibrium at each evaluated position. This allowed for the computation of total solid solution interstitial content, which was expressed in atomic fraction. Plots of as-quenched hardness against the square root of the computed interstitial content, i.e. the sum of solution carbon and the remaining nitrogen, show the complementary character of these elements in determining the mechanical properties of the materials prior to stress relief treatment. Tempering of carbon-nitrogen martensite resulted in hardness drop to a lesser degree than the one measured on carbon martensite with equivalent interstitial content. In order to investigate this behavior, transmission electron microscopy (TEM) analyses were performed. Results showed the precipitation of two morphologies of Fe 16 N 2 in the nitrogen-rich case and image analysis confirmed the simulated fraction of nitrides.